extensions: Check |enable_extensions| flag before depending on extension code.

apps/apps.gypi

 # 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.
 
 {
   'targets': [
     {
       # GN version: //apps
       'target_name': 'apps',
       'type': 'static_library',
@@ skipping 57 matching lines @@
               'browser_extensions',
               'common/extensions/api/api.gyp:chrome_api',
             ],
             'sources/': [
               ['exclude', '.*'],
             ],
           }
         ],
         ['toolkit_views==1', {
           'dependencies': [
-            '../extensions/extensions.gyp:extensions_browser',
             '../ui/strings/ui_strings.gyp:ui_strings',
             '../ui/views/views.gyp:views',
           ],
         }, {  # toolkit_views==0
           'sources/': [
             ['exclude', 'ui/views/'],
           ],
         }],
+        ['toolkit_views==1 and enable_extensions==1', {
+          'dependencies': [
+            '../extensions/extensions.gyp:extensions_browser',
+          ],
+        }],
       ],
       # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
       'msvs_disabled_warnings': [ 4267, ],
     },
   ],  # targets
 }