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

chrome/browser/extensions/settings/weak_unlimited_settings_storage.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_WEAK_UNLIMITED_SETTINGS_STORAGE_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_WEAK_UNLIMITED_SETTINGS_STORAGE_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "chrome/browser/extensions/settings/settings_storage.h"
 
 namespace extensions {
 
 // A SettingsStorage decorator which makes calls through |Set| ignore quota.
 // "Weak" because ownership of the delegate isn't taken; this is designed to be
 // temporarily attached to storage areas.
 class WeakUnlimitedSettingsStorage : public SettingsStorage {
  public:
   // Ownership of |delegate| NOT taken.
   explicit WeakUnlimitedSettingsStorage(SettingsStorage* delegate);
 
   virtual ~WeakUnlimitedSettingsStorage();
 
   // 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;
   virtual WriteResult Set(
       WriteOptions options,
       const std::string& key,
       const Value& value) OVERRIDE;
   virtual WriteResult Set(
       WriteOptions options, const DictionaryValue& values) OVERRIDE;
   virtual WriteResult Remove(const std::string& key) OVERRIDE;
   virtual WriteResult Remove(const std::vector<std::string>& keys) OVERRIDE;
   virtual WriteResult Clear() OVERRIDE;
 
  private:
   // The delegate storage area, NOT OWNED.
   SettingsStorage* const delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(WeakUnlimitedSettingsStorage);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_WEAK_UNLIMITED_SETTINGS_STORAGE_H_