Added Vp9 simulcast tests.

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 82afd9e6c0fa6437910d55e17d59d39a5074bfc5..173e9a0a8ed182af924218a942eb1eb708754ec4 100644
--- a/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
+++ b/webrtc/modules/video_coding/codecs/vp9/vp9_impl.cc
@@ -251,8 +251,8 @@ int VP9EncoderImpl::InitEncode(const VideoCodec* inst,
   if (inst->VP9().numberOfTemporalLayers > 3) {
     return WEBRTC_VIDEO_CODEC_ERR_PARAMETER;
   }
-  // libvpx currently supports only one or two spatial layers.
-  if (inst->VP9().numberOfSpatialLayers > 2) {
+  // libvpx probably does not support more than 3 spatial layers.
+  if (inst->VP9().numberOfSpatialLayers > 3) {
     return WEBRTC_VIDEO_CODEC_ERR_PARAMETER;
   }