Add ExtensionsTestBrowserContext class for test.

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.
 {
   'targets': [
     {
       # This target contains mocks and test utilities that don't belong in
       # production libraries but are used by more than one test executable.
       'target_name': 'test_support_common',
       'type': 'static_library',
@@ skipping 12 matching lines @@
         '../components/components.gyp:gcm_driver_test_support',
         '../components/components.gyp:history_core_test_support',
         '../components/components.gyp:invalidation_test_support',
         '../components/components.gyp:metrics_test_support',
         '../components/components.gyp:password_manager_core_browser_test_support',
         '../components/components.gyp:pref_registry_test_support',
         '../components/components.gyp:signin_core_browser_test_support',
         '../components/components.gyp:sync_driver_test_support',
         '../content/content.gyp:content_app_both',
         '../content/content_shell_and_tests.gyp:test_support_content',
+        '../extensions/extensions.gyp:extensions_test_support',
         '../net/net.gyp:net',
         '../net/net.gyp:net_test_support',
         '../skia/skia.gyp:skia',
         '../sql/sql.gyp:sql',
         '../sql/sql.gyp:test_support_sql',
         '../sync/sync.gyp:sync',
         '../testing/gmock.gyp:gmock',
         '../testing/gtest.gyp:gtest',
         '../third_party/zlib/zlib.gyp:zlib',
         '../ui/message_center/message_center.gyp:message_center_test_support',
@@ skipping 2874 matching lines @@
             'unit_tests.isolate',
           ],
           'sources': [
             'unit_tests.isolate',
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }