Implement NullVideoDecoder to avoid crash on unsupported decoders.

talk/media/webrtc/webrtcvideoengine2.cc

Index: talk/media/webrtc/webrtcvideoengine2.cc
diff --git a/talk/media/webrtc/webrtcvideoengine2.cc b/talk/media/webrtc/webrtcvideoengine2.cc
index d2db434aae7a978fdc85c9465e00416bb82cd8e7..353badcda854d63a6dff1b8cda2f1c1666f2775a 100644
--- a/talk/media/webrtc/webrtcvideoengine2.cc
+++ b/talk/media/webrtc/webrtcvideoengine2.cc
@@ -2258,10 +2258,9 @@ WebRtcVideoChannel2::WebRtcVideoReceiveStream::CreateOrReuseVideoDecoder(
         webrtc::VideoDecoder::Create(webrtc::VideoDecoder::kH264), type, false);
   }
 
-  // This shouldn't happen, we should not be trying to create something we don't
-  // support.
-  RTC_DCHECK(false);
-  return AllocatedDecoder(NULL, webrtc::kVideoCodecUnknown, false);
+  return AllocatedDecoder(
+      webrtc::VideoDecoder::Create(webrtc::VideoDecoder::kUnsupportedCodec),
+      webrtc::kVideoCodecUnknown, false);
 }
 
 void WebRtcVideoChannel2::WebRtcVideoReceiveStream::ConfigureCodecs(