Replace the index mapping of wallpaper picker UI and hard coded wallpaper index in C++

chrome/chrome_browser.gypi

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'browser',
       'type': 'static_library',
       'variables': { 'enable_wexit_time_destructors': 1, },
@@ skipping 3766 matching lines @@
         'browser/ui/webui/options2/chromeos/set_wallpaper_options_handler2.cc',
         'browser/ui/webui/options2/chromeos/set_wallpaper_options_handler2.h',
         'browser/ui/webui/options2/chromeos/stats_options_handler2.cc',
         'browser/ui/webui/options2/chromeos/stats_options_handler2.h',
         'browser/ui/webui/options2/chromeos/system_settings_provider2.cc',
         'browser/ui/webui/options2/chromeos/system_settings_provider2.h',
         'browser/ui/webui/options2/chromeos/user_image_source2.cc',
         'browser/ui/webui/options2/chromeos/user_image_source2.h',
         'browser/ui/webui/options2/chromeos/virtual_keyboard_manager_handler2.cc',
         'browser/ui/webui/options2/chromeos/virtual_keyboard_manager_handler2.h',
+        'browser/ui/webui/options2/chromeos/wallpaper_thumbnail_source2.cc',
+        'browser/ui/webui/options2/chromeos/wallpaper_thumbnail_source2.h',
         'browser/ui/webui/options2/clear_browser_data_handler2.cc',
         'browser/ui/webui/options2/clear_browser_data_handler2.h',
         'browser/ui/webui/options2/content_settings_handler2.cc',
         'browser/ui/webui/options2/content_settings_handler2.h',
         'browser/ui/webui/options2/cookies_view_handler2.cc',
         'browser/ui/webui/options2/cookies_view_handler2.h',
         'browser/ui/webui/options2/core_options_handler2.cc',
         'browser/ui/webui/options2/core_options_handler2.h',
         'browser/ui/webui/options2/font_settings_handler2.cc',
         'browser/ui/webui/options2/font_settings_handler2.h',
@@ skipping 1453 matching lines @@
       'type': 'static_library',
       'sources': [ 'browser/chromeos/gdata/gdata.proto' ],
       'variables': {
         'proto_in_dir': 'browser/chromeos/gdata',
         'proto_out_dir': 'chrome/browser/chromeos/gdata',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ],
 }