[Extensions] Validate file existence for chrome url overrides

chrome/chrome_tests_unit.gypi

 # Copyright 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.
 {
   'variables': {
     'chrome_unit_tests_sources': [
       # histograms.xml is analyzed by AboutFlagsHistogramTest, so this
       # dependency is needed to make commit bots run unit_tests on
       # histograms.xml changes.
       '../tools/metrics/histograms/histograms.xml',
@@ skipping 609 matching lines @@
       'browser/sync_file_system/sync_process_runner_unittest.cc',
       'browser/sync_file_system/syncable_file_system_util_unittest.cc',
       'browser/web_applications/web_app_mac_unittest.mm',
       'browser/web_applications/web_app_unittest.cc',
       'common/extensions/api/commands/commands_manifest_unittest.cc',
       'common/extensions/api/common_extension_api_unittest.cc',
       'common/extensions/api/extension_action/browser_action_manifest_unittest.cc',
       'common/extensions/api/extension_action/page_action_manifest_unittest.cc',
       'common/extensions/api/storage/storage_schema_manifest_handler_unittest.cc',
       'common/extensions/chrome_extensions_client_unittest.cc',
+      'common/extensions/chrome_manifest_url_handlers_unittest.cc',
       'common/extensions/command_unittest.cc',
       'common/extensions/extension_icon_set_unittest.cc',
       'common/extensions/extension_unittest.cc',
       'common/extensions/feature_switch_unittest.cc',
       'common/extensions/features/chrome_channel_feature_filter_unittest.cc',
       'common/extensions/manifest_handlers/automation_unittest.cc',
       'common/extensions/manifest_handlers/content_scripts_manifest_unittest.cc',
       'common/extensions/manifest_handlers/exclude_matches_manifest_unittest.cc',
       'common/extensions/manifest_handlers/settings_overrides_handler_unittest.cc',
       'common/extensions/manifest_handlers/ui_overrides_handler_unittest.cc',
@@ skipping 2348 matching lines @@
               'dependencies': [
                 '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
               ],
             }],
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }