Make sure WebRTC works without libvpx VP9 support.

webrtc/modules/video_coding/codecs/vp9/include/vp9.h

 /*
  *  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.
  *
  */
 
 #ifndef WEBRTC_MODULES_VIDEO_CODING_CODECS_VP9_INCLUDE_VP9_H_
 #define WEBRTC_MODULES_VIDEO_CODING_CODECS_VP9_INCLUDE_VP9_H_
 
 #include "webrtc/modules/video_coding/include/video_codec_interface.h"
 
 namespace webrtc {
 
 class VP9Encoder : public VideoEncoder {
  public:
+  static bool IsSupported();
   static VP9Encoder* Create();
 
   virtual ~VP9Encoder() {}
 };
 
 class VP9Decoder : public VideoDecoder {
  public:
+  static bool IsSupported();
   static VP9Decoder* Create();
 
   virtual ~VP9Decoder() {}
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_CODECS_VP9_INCLUDE_VP9_H_