Make ui gn check pass. Misc GN build improvements.

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 857 matching lines @@
         'common/api/api.gyp:extensions_api',
         'extensions_browser',
         'extensions_common',
         'extensions_resources.gyp:extensions_resources',
       ],
       'include_dirs': [
         '..',
         '<(SHARED_INTERMEDIATE_DIR)',
       ],
       'sources': [
+        # Note: sources list duplicated in GN build.
         'browser/api/dns/mock_host_resolver_creator.cc',
         'browser/api/dns/mock_host_resolver_creator.h',
         'browser/api/storage/settings_test_util.cc',
         'browser/api/storage/settings_test_util.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',
@@ skipping 194 matching lines @@
           'browser/api/cast_channel/logging.proto'
       ],
       'variables': {
           'proto_in_dir': 'browser/api/cast_channel',
           'proto_out_dir': 'extensions/browser/api/cast_channel',
       },
       'includes': [ '../build/protoc.gypi' ]
     },
   ]
 }