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

chrome/browser/extensions/settings/settings_frontend.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_FRONTEND_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_FRONTEND_H_
 #pragma once
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list_threadsafe.h"
 #include "chrome/browser/extensions/settings/settings_leveldb_storage.h"
 #include "chrome/browser/extensions/settings/settings_namespace.h"
 #include "chrome/browser/extensions/settings/settings_observer.h"
+#include "chrome/browser/extensions/settings/settings_storage_quota_enforcer.h"
 #include "chrome/browser/sync/api/syncable_service.h"
 
 class Profile;
 
 namespace extensions {
 
 class SettingsStorage;
 
 // The component of extension settings which runs on the UI thread, as opposed
 // to SettingsBackend which lives on the FILE thread.
 // All public methods must be called on the UI thread.
 class SettingsFrontend {
  public:
-  // Creates with the default factory.  Ownership of |profile| not taken.
+  // Quota limits for sync and local. Make these public for tests.
+  static const SettingsStorageQuotaEnforcer::Limits kSyncQuota;
+  static const SettingsStorageQuotaEnforcer::Limits kLocalQuota;
+
+  // Creates with the default factory. Ownership of |profile| not taken.
   static SettingsFrontend* Create(Profile* profile);
 
+  // Creates with a specific factory |storage_factory| (presumably for tests).
+  // Ownership of |profile| not taken.
   static SettingsFrontend* Create(
       const scoped_refptr<SettingsStorageFactory>& storage_factory,
       // Owership NOT taken.
       Profile* profile);
 
   virtual ~SettingsFrontend();
 
   typedef base::Callback<void(SyncableService*)> SyncableServiceCallback;
   typedef base::Callback<void(SettingsStorage*)> StorageCallback;
 
@@ skipping 44 matching lines @@
     scoped_refptr<BackendWrapper> extension;
   };
   std::map<settings_namespace::Namespace, BackendWrappers> backends_;
 
   DISALLOW_COPY_AND_ASSIGN(SettingsFrontend);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_FRONTEND_H_