[Fallback Icons] Refactor FallbackIconService to be a BrowserContext-level singleton

components/favicon.gypi

 # 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.
 
 {
   'targets': [
     {
       # GN version: //components/favicon/core
       'target_name': 'favicon_core',
       'type': 'static_library',
       'dependencies': [
         '../ui/gfx/gfx.gyp:gfx_geometry',
         'favicon_base',
         'keyed_service_core',
       ],
       'sources': [
         # Note: sources list duplicated in GN build.
+        'favicon/core/browser/fallback_icon_client.h',
         'favicon/core/browser/favicon_client.h',
         'favicon/core/favicon_driver.h',
         'favicon/core/favicon_url.cc',
         'favicon/core/favicon_url.h',
       ],
       'include_dirs': [
         '..',
       ],
     },
   ],
 }