Make sure WebRTC works without libvpx VP9 support.

Wires up existing libvpx_build_vp9==0 GYP flag into WebRTC and makes VP9
optional. Change is GYP only for now since libvpx's GN files build VP9
unconditionally.

BUG=webrtc:5884
R=kjellander@webrtc.org

Review URL: https://codereview.webrtc.org/1970343002 .

Cr-Commit-Position: refs/heads/master@{#12741}

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

@@ -18,6 +18,7 @@ namespace webrtc {
 
 class VP9Encoder : public VideoEncoder {
  public:
+  static bool IsSupported();
   static VP9Encoder* Create();
 
   virtual ~VP9Encoder() {}
@@ -25,6 +26,7 @@ class VP9Encoder : public VideoEncoder {
 
 class VP9Decoder : public VideoDecoder {
  public:
+  static bool IsSupported();
   static VP9Decoder* Create();
 
   virtual ~VP9Decoder() {}