Extension Storage API: expose storage quota information to extensions, via:

chrome/common/extensions/api/storage.json

 [
   {
     "namespace": "storage",
     "types": [
       {
         "id": "StorageChange",
         "type": "object",
         "properties": {
           "oldValue": {
             "type": "any",
             "description": "The old value of the item, if there was an old value.",
             "optional": true
           },
           "newValue": {
             "type": "any",
             "description": "The new value of the item, if there is a new value.",
             "optional": true
           }
         }
       },
       {
-        "id": "StorageNamespace",
+        "id": "StorageArea",
         "type": "object",
         "functions": [
           {
             "name": "get",
             "unprivileged": true,
             "type": "function",
             "description": "Gets one or more items from storage.",
             "parameters": [
               {
                 "name": "keys",
@@ skipping 78 matching lines @@
             "description": "Removes all items from storage.",
             "parameters": [
               {
                 "name": "callback",
                 "type": "function",
                 "description": "Callback on success, or on failure (in which case lastError will be set).",
                 "parameters": [],
                 "optional": true
               }
             ]
+          },
+          {
+            "name": "getBytesInUse",
+            "unprivileged": true,
+            "type": "function",
+            "description": "Get the amount of space being used in storage, in bytes.",
+            "parameters": [
+              {
+                "name": "callback",
+                "type": "function",
+                "description": "Callback with the amount of space being used by storage, or on failure (in which case lastError will be set).",
+                "parameters": [
+                  {
+                    "name": "bytesInUse",
+                    "type": "integer",
+                    "description": "Amount of space being used in storage, in bytes."
+                  }
+                ]
+              }
+            ]
           }
         ]
       }
     ],
     "events": [
       {
         "name": "onChanged",
         "unprivileged": true,
         "type": "function",
         "description": "Fired when one or more items change.",
         "parameters": [
           {
             "name": "changes",
             "type": "object",
             "properties": {},
             "additionalProperties": { "$ref": "StorageChange" },
             "description": "Object mapping each key that changed to its corresponding <a href='#type-StorageChange'>StorageChange</a> for that item."
           },
           {
             "name": "namespace",
             "type": "string",
             "description": "The namespace (\"sync\" or \"local\") of the storage area the changes are for."
           }
         ]
       }
     ],
     "properties": {
       "sync": {
-        "$ref": "StorageNamespace",
+        "$ref": "StorageArea",
         "description": "Items under the \"sync\" namespace are synced using Chrome Sync.",
         "unprivileged": true,
-        "value": [ "sync" ]
+        "value": [ "sync" ],
+        "properties": {
+          "QUOTA_BYTES": {

[Matt Perry, 2012/01/24 21:54:59]

What's our style for API constants? I don't think I've seen the ALL_CAPS style
before.

[not at google - send to devlin, 2012/01/24 21:56:48]

Ah, I was basing it off that WINDOW_ID_NONE variable in windows.json (because
I'd seen it before for a different reason). Maverick constant?

[Matt Perry, 2012/01/24 22:02:19]

Oh OK, sounds like there's precedent. Carry on, then.

+            "type": "integer",
+            "value": "102400",
+            "unprivileged": true,
+            "description": "The maximum total amount (in bytes) of data that can be stored in sync storage."
+          },
+          "QUOTA_BYTES_PER_ITEM": {
+            "type": "integer",
+            "value": "2048",
+            "unprivileged": true,
+            "description": "The maximum size (in bytes) of each individual item in sync storage."
+          },
+          "MAX_ITEMS": {
+            "type": "integer",
+            "value": "512",
+            "unprivileged": true,
+            "description": "The maximum number of items that can be stored in sync storage."
+          }
+        }
       },
       "local": {
-        "$ref": "StorageNamespace",
+        "$ref": "StorageArea",
         "description": "Items under the \"local\" namespace are local to each machine.",
         "unprivileged": true,
-        "value": [ "local" ]
+        "value": [ "local" ],
+        "properties": {
+          "QUOTA_BYTES": {
+            "type": "integer",
+            "value": "5120000",
+            "unprivileged": true,
+            "description": "The maximum amount (in bytes) of data that can be stored in local storage. This value may be ignored if the extension has the <tt>unlimitedStorage</tt> permission."
+          }
+        }
       }
     }
   }
 ]