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

chrome/browser/extensions/settings/settings_leveldb_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_SETTINGS_LEVELDB_STORAGE_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_LEVELDB_STORAGE_H_
 #pragma once
 
 #include <string>
 
@@ skipping 20 matching lines @@
 
    private:
     // SettingsStorageFactory is refcounted.
     virtual ~Factory() {}
   };
 
   // Must be deleted on the FILE thread.
   virtual ~SettingsLeveldbStorage();
 
   // 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;
@@ skipping 22 matching lines @@
 
   // leveldb backend.
   scoped_ptr<leveldb::DB> db_;
 
   DISALLOW_COPY_AND_ASSIGN(SettingsLeveldbStorage);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_LEVELDB_STORAGE_H_