Extract a MockExtensionSystem to be used with TestExtensionsBrowserClient.

extensions/extensions.gyp

 # Copyright 2013 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': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 701 matching lines @@
         '..',
         '<(SHARED_INTERMEDIATE_DIR)',
       ],
       'sources': [
         'browser/api/dns/mock_host_resolver_creator.cc',
         'browser/api/dns/mock_host_resolver_creator.h',
         'browser/api_test_utils.cc',
         'browser/api_test_utils.h',
         'browser/extensions_test.cc',
         'browser/extensions_test.h',
+        'browser/mock_extension_system.cc',
+        'browser/mock_extension_system.h',
         'browser/test_extensions_browser_client.cc',
         'browser/test_extensions_browser_client.h',
         'browser/test_management_policy.cc',
         'browser/test_management_policy.h',
         'browser/test_runtime_api_delegate.cc',
         'browser/test_runtime_api_delegate.h',
         'common/extension_builder.cc',
         'common/extension_builder.h',
         'common/test_util.cc',
         'common/test_util.h',
@@ skipping 154 matching lines @@
       'type': 'static_library',
       'sources': [ 'browser/api/cast_channel/cast_channel.proto' ],
       'variables': {
           'proto_in_dir': 'browser/api/cast_channel',
           'proto_out_dir': 'extensions/browser/api/cast_channel',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ]
 }