Extensions: Do not depend on device_serial_mojo target when extensions are turned off.

extensions/extensions_resources.gyp

 # Copyright 2014 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': [
     {
       'target_name': 'extensions_resources',
       'type': 'none',
-      'dependencies': [
-        '../device/serial/serial.gyp:device_serial_mojo',
-      ],
       'variables': {
         'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/extensions',
       },
       'actions': [
         {
           'action_name': 'generate_extensions_resources',
           'variables': {
             'grit_grd_file': 'extensions_resources.grd',
           },
           'includes': [ '../build/grit_action.gypi' ],
         },
         {
           'action_name': 'generate_extensions_renderer_resources',
           'variables': {
             'grit_grd_file': 'renderer/resources/extensions_renderer_resources.grd',
             'grit_additional_defines': [
               '-E', 'mojom_root=<(SHARED_INTERMEDIATE_DIR)',
             ],
           },
           'includes': [ '../build/grit_action.gypi' ],
         },
       ],
       'includes': [ '../build/grit_target.gypi' ],
+      'conditions': [
+        ['enable_extensions==1', {
+          'dependencies': [
+            '../device/serial/serial.gyp:device_serial_mojo',
+          ],
+        }],
+      ],
     }
   ]
 }