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

chrome/browser/extensions/settings/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_SETTINGS_STORAGE_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_STORAGE_H_
 #pragma once
 
 #include <set>
 
@@ skipping 82 matching lines @@
   enum WriteOptions {
     // Callers should usually use this.
     DEFAULTS,
 
     // Ignore any quota restrictions.
     IGNORE_QUOTA,
   };
 
   virtual ~SettingsStorage() {}
 
+  // Gets the amount of space being used by a single value, in bytes.
+  virtual size_t GetBytesInUse(const std::string& key) = 0;
+
+  // Gets the total amount of space being used by multiple values, in bytes.
+  virtual size_t GetBytesInUse(const std::vector<std::string>& keys) = 0;
+
+  // Gets the total amount of space being used by this storage area, in bytes.
+  virtual size_t GetBytesInUse() = 0;
+
   // Gets a single value from storage.
   virtual ReadResult Get(const std::string& key) = 0;
 
   // Gets multiple values from storage.
   virtual ReadResult Get(const std::vector<std::string>& keys) = 0;
 
   // Gets all values from storage.
   virtual ReadResult Get() = 0;
 
   // Sets a single key to a new value.
@@ skipping 10 matching lines @@
   // Removes multiple keys from the storage.
   virtual WriteResult Remove(const std::vector<std::string>& keys) = 0;
 
   // Clears the storage.
   virtual WriteResult Clear() = 0;
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_SETTINGS_STORAGE_H_