Remove dependency on BrowserThread::FILE from Sync code

chrome/browser/extensions/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
 import("//extensions/features/features.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
@@ skipping 356 matching lines @@
     "api/settings_private/settings_private_event_router_factory.cc",
     "api/settings_private/settings_private_event_router_factory.h",
     "api/signed_in_devices/id_mapping_helper.cc",
     "api/signed_in_devices/id_mapping_helper.h",
     "api/signed_in_devices/signed_in_devices_api.cc",
     "api/signed_in_devices/signed_in_devices_api.h",
     "api/signed_in_devices/signed_in_devices_manager.cc",
     "api/signed_in_devices/signed_in_devices_manager.h",
     "api/spellcheck/spellcheck_api.cc",
     "api/spellcheck/spellcheck_api.h",
+    "api/storage/backend_task_runner.cc",
+    "api/storage/backend_task_runner.h",
     "api/storage/managed_value_store_cache.cc",
     "api/storage/managed_value_store_cache.h",
     "api/storage/policy_value_store.cc",
     "api/storage/policy_value_store.h",
     "api/storage/setting_sync_data.cc",
     "api/storage/setting_sync_data.h",
     "api/storage/settings_sync_processor.cc",
     "api/storage/settings_sync_processor.h",
     "api/storage/settings_sync_util.cc",
     "api/storage/settings_sync_util.h",
@@ skipping 784 matching lines @@
       "api/braille_display_private/brlapi_connection.cc",
       "api/braille_display_private/brlapi_connection.h",
       "api/braille_display_private/brlapi_keycode_map.cc",
       "api/braille_display_private/brlapi_keycode_map.h",
     ]
     deps += [ "//build/linux/libbrlapi" ]
   } else {
     sources += [ "api/braille_display_private/braille_controller_stub.cc" ]
   }
 }