Make sure WebRTC works without libvpx VP9 support.

webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc

Index: webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
diff --git a/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc b/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
index ecd185856f8f054e3296eec76550a54630a0421a..21d9d3dcfb5fc019bb946924a1f37e23b6f58a17 100644
--- a/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
+++ b/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
@@ -47,6 +47,10 @@ int GetCpuSpeed(int width, int height) {
 #endif
 }
 
+bool VP9Encoder::IsSupported() {
+  return true;
+}
+
 VP9Encoder* VP9Encoder::Create() {
   return new VP9EncoderImpl();
 }
@@ -820,6 +824,10 @@ const char* VP9EncoderImpl::ImplementationName() const {
   return "libvpx";
 }
 
+bool VP9Decoder::IsSupported() {
+  return true;
+}
+
 VP9Decoder* VP9Decoder::Create() {
   return new VP9DecoderImpl();
 }