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

chrome/browser/extensions/settings/syncable_settings_storage.h

Index: chrome/browser/extensions/settings/syncable_settings_storage.h
diff --git a/chrome/browser/extensions/settings/syncable_settings_storage.h b/chrome/browser/extensions/settings/syncable_settings_storage.h
index d650229abaebc07e8d8da9412c13d206173b3bbe..8c8b1e0169f0715f944ba172f9f08c779bcb516d 100644
--- a/chrome/browser/extensions/settings/syncable_settings_storage.h
+++ b/chrome/browser/extensions/settings/syncable_settings_storage.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -31,6 +31,9 @@ class SyncableSettingsStorage : public SettingsStorage {
   virtual ~SyncableSettingsStorage();
 
   // SettingsStorage implementation.
+  virtual size_t GetBytesInUse(const std::string& key) OVERRIDE;
+  virtual size_t GetBytesInUse(const std::vector<std::string>& keys) OVERRIDE;
+  virtual size_t GetBytesInUse() OVERRIDE;
   virtual ReadResult Get(const std::string& key) OVERRIDE;
   virtual ReadResult Get(const std::vector<std::string>& keys) OVERRIDE;
   virtual ReadResult Get() OVERRIDE;