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

chrome/common/extensions/api/storage.json

Index: chrome/common/extensions/api/storage.json
diff --git a/chrome/common/extensions/api/storage.json b/chrome/common/extensions/api/storage.json
index a52e2171aec1ceba0c6de7d35adca4b0c0954dee..fbf45f0fd27b6629dcaf121a6d5f6c800e47fe4e 100644
--- a/chrome/common/extensions/api/storage.json
+++ b/chrome/common/extensions/api/storage.json
@@ -19,7 +19,7 @@
         }
       },
       {
-        "id": "StorageNamespace",
+        "id": "StorageArea",
         "type": "object",
         "functions": [
           {
@@ -118,6 +118,26 @@
                 "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."
+                  }
+                ]
+              }
+            ]
           }
         ]
       }
@@ -146,16 +166,44 @@
     ],
     "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."
+          }
+        }
       }
     }
   }