Add support for extensions_unittests to the GN build

extensions/extensions_tests.gyp

 {
   'variables': {
     # Product name is used for Mac bundle.
     'app_shell_product_name': 'App Shell',
     # The version is high enough to be supported by Omaha (at least 31)
     # but fake enough to be obviously not a Chrome release.
     'app_shell_version': '38.1234.5678.9',
     'chromium_code': 1,
   },
   'targets': [
@@ skipping 11 matching lines @@
         '../device/serial/serial.gyp:device_serial_test_util',
         '../mojo/mojo_base.gyp:mojo_environment_chromium',
         '../testing/gmock.gyp:gmock',
         '../testing/gtest.gyp:gtest',
         '../third_party/leveldatabase/leveldatabase.gyp:leveldatabase',
         '../third_party/mojo/mojo_edk.gyp:mojo_js_lib',
         '../third_party/mojo/mojo_edk.gyp:mojo_system_impl',
         '../third_party/mojo/mojo_public.gyp:mojo_cpp_bindings',
         '../third_party/mojo/mojo_public.gyp:mojo_application_bindings',
         'common/api/api.gyp:cast_channel_proto',
+        'extensions.gyp:extensions_browser',
         'extensions.gyp:extensions_common',
         'extensions.gyp:extensions_renderer',
         'extensions.gyp:extensions_shell_and_test_pak',
         'extensions.gyp:extensions_test_support',
         'extensions.gyp:extensions_utility',
         'extensions_resources.gyp:extensions_resources',
         'extensions_strings.gyp:extensions_strings',
       ],
       # Needed for third_party libraries like leveldb.
       'include_dirs': [
@@ skipping 206 matching lines @@
             '../build/isolate.gypi',
           ],
           'sources': [
             'extensions_browsertests.isolate',
           ],
         },
       ],
     }],
   ],
 }