Add 1st cut of FileSystemUsageTracker that tracks the usage changes in FileSystem API.

webkit/fileapi/webkit_fileapi.gypi

 # Copyright (c) 2010 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.
 
 {
   'targets': [
     {
       'target_name': 'fileapi',
       'type': '<(library)',
       'msvs_guid': '40B53211-03ED-4932-8D53-52B172599DFE',
       'dependencies': [
         '<(DEPTH)/app/app.gyp:app_base',
         '<(DEPTH)/base/base.gyp:base',
         '<(DEPTH)/net/net.gyp:net',
       ],
       'sources': [
         'file_system_callback_dispatcher.h',
         'file_system_operation.cc',
         'file_system_operation.h',
         'file_system_path_manager.cc',
         'file_system_path_manager.h',
         'file_system_quota_manager.cc',
         'file_system_quota_manager.h',
         'file_system_types.h',
+        'file_system_usage_tracker.h',
+        'file_system_usage_tracker.cc',
         'file_writer_delegate.cc',
         'file_writer_delegate.h',
         'sandboxed_file_system_context.cc',
         'sandboxed_file_system_context.h',
         'sandboxed_file_system_operation.cc',
         'sandboxed_file_system_operation.h',
         'webfilewriter_base.cc',
         'webfilewriter_base.h',
       ],
       'conditions': [
         ['inside_chromium_build==0', {
           'dependencies': [
             '<(DEPTH)/webkit/support/setup_third_party.gyp:third_party_headers',
           ],
         }],
       ],
     },
   ],
 }