Make sure WebRTC works without libvpx VP9 support.

webrtc/modules/video_coding/codecs/vp9/vp9.gyp

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'includes': [
     '../../../../build/common.gypi',
   ],
   'targets': [
     {
       'target_name': 'webrtc_vp9',
       'type': 'static_library',
       'conditions': [
         ['build_libvpx==1', {
           'dependencies': [
             '<(libvpx_dir)/libvpx.gyp:libvpx',
           ],
         }],
+        ['libvpx_build_vp9==1', {
+          'sources': [
+            'screenshare_layers.cc',
+            'screenshare_layers.h',
+            'vp9_frame_buffer_pool.cc',
+            'vp9_frame_buffer_pool.h',
+            'vp9_impl.cc',
+            'vp9_impl.h',
+          ],
+        }, {
+          'sources': [
+            'vp9_noop.cc',
+          ],
+        }
+        ],
       ],
       'dependencies': [
         '<(webrtc_root)/common_video/common_video.gyp:common_video',
         '<(webrtc_root)/modules/video_coding/utility/video_coding_utility.gyp:video_coding_utility',
         '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
       ],
       'sources': [
         'include/vp9.h',
-        'screenshare_layers.cc',
-        'screenshare_layers.h',
-        'vp9_frame_buffer_pool.cc',
-        'vp9_frame_buffer_pool.h',
-        'vp9_impl.cc',
-        'vp9_impl.h',
       ],
     },
   ],
 }