extensions: Add GN version of extensions_shell_and_test_pak and unittests target.

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 834 matching lines @@
       'conditions': [
         ['OS=="win" and win_use_allocator_shim==1', {
           'dependencies': [
             '../base/allocator/allocator.gyp:allocator',
           ],
         }],
       ],
     },
     {
       # Protobuf compiler / generator for chrome.cast.channel-related protocol buffers.
-      # GN version: //extensions/browser/api/cast_channel/BUILD.gn
+      # GN version: //extensions/browser/api/cast_channel:cast_channel_proto
       'target_name': 'cast_channel_proto',
       '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' ]
     },
   ]
 }