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

chrome/browser/extensions/settings/settings_api.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_API_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_API_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
@@ skipping 81 matching lines @@
  public:
   DECLARE_EXTENSION_FUNCTION_NAME("storage.clear");
 
  protected:
   virtual bool RunWithStorage(SettingsStorage* storage) OVERRIDE;
 
   virtual void GetQuotaLimitHeuristics(
       QuotaLimitHeuristics* heuristics) const OVERRIDE;
 };
 
+class GetBytesInUseSettingsFunction : public SettingsFunction {
+ public:
+  DECLARE_EXTENSION_FUNCTION_NAME("storage.getBytesInUse");
+
+ protected:
+  virtual bool RunWithStorage(SettingsStorage* storage) OVERRIDE;
+};
+
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_API_H_