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

chrome/test/data/extensions/api_test/settings/simple_test/background.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 function test(stage0) {
   var apis = [
     chrome.storage.sync,
     chrome.storage.local
   ];
   apis.forEach(function(api) {
@@ skipping 273 matching lines @@
     function stage8(settings) {
       chrome.test.assertEq({
         'foo': 'defaultBar',
         'baz': {}
       }, settings);
       this.succeed();
     }
     test(stage0);
   },
 
+
+  function quota() {
+    // Just check that the constants are defined; no need to be forced to
+    // update them here as well if/when they change.
+    chrome.test.assertTrue(chrome.storage.sync.QUOTA_BYTES > 0);
+    chrome.test.assertTrue(chrome.storage.sync.QUOTA_BYTES_PER_ITEM > 0);
+    chrome.test.assertTrue(chrome.storage.sync.MAX_ITEMS > 0);
+
+    chrome.test.assertTrue(chrome.storage.local.QUOTA_BYTES > 0);
+    chrome.test.assertEq('undefined',
+                         typeof chrome.storage.local.QUOTA_BYTES_PER_ITEM);
+    chrome.test.assertEq('undefined',
+                         typeof chrome.storage.local.MAX_ITEMS);
+
+    var area = chrome.storage.sync;
+    function stage0() {
+      area.getBytesInUse(stage1);
+    }
+    function stage1(bytesInUse) {
+      chrome.test.assertEq(0, bytesInUse);
+      area.set({ a: 42 }, stage2);
+    }
+    function stage2() {
+      area.getBytesInUse(stage3);
+    }
+    function stage3(bytesInUse) {
+      chrome.test.assertEq(3, bytesInUse);
+      chrome.test.succeed();
+    }
+    area.clear(stage0);
+  },
+
+  // NOTE: throttling test must come last, since each test runs with a single
+  // quota.
   function throttling() {
     // We can only really test one of the namespaces since they will all get
     // throttled together.
     var api = chrome.storage.sync;
 
-    // Should get throttled after 1000 calls.
+    // Should get throttled after 1000 calls (though in reality will be fewer
+    // due to previous tests).
     var maxRequests = 1001;
 
     function next() {
       api.clear((--maxRequests > 0) ? next : done);
     }
     function done() {
       chrome.test.assertEq(
           "This request exceeds available quota.",
           chrome.extension.lastError.message);
       chrome.test.succeed();
     }
     api.clear(next);
   }
+
 ]);