Enable RTCVideoDecoder SW H264 fallback only when it is available

Extensions have HW H264 encode enabled in 51. However SW implementations
of H264 isn't enabled by default. So, we need to be careful about SW fallback
as we cannot do it when it isn't available.

BUG=615513

Review-Url: https://codereview.chromium.org/2024303003
Cr-Commit-Position: refs/heads/master@{#397856}

content/renderer/media/rtc_video_decoder.cc

@@ -21,6 +21,7 @@
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/webrtc/base/bind.h"
 #include "third_party/webrtc/base/refcount.h"
+#include "third_party/webrtc/modules/video_coding/codecs/h264/include/h264.h"
 #include "third_party/webrtc/video_frame.h"
 
 #if defined(OS_WIN)
@@ -180,7 +181,10 @@ int32_t RTCVideoDecoder::Decode(
   if (state_ == DECODE_ERROR) {
     LOG(ERROR) << "Decoding error occurred.";
     // Try reseting the session up to |kNumVDAErrorsHandled| times.
-    if (vda_error_counter_ > kNumVDAErrorsBeforeSWFallback) {
+    // Check if SW H264 implementation is available before falling back.
+    if (vda_error_counter_ > kNumVDAErrorsBeforeSWFallback &&
+        (video_codec_type_ != webrtc::kVideoCodecH264 ||
+         webrtc::H264Decoder::IsSupported())) {
       DLOG(ERROR) << vda_error_counter_
                   << " errors reported by VDA, falling back to software decode";
       return WEBRTC_VIDEO_CODEC_FALLBACK_SOFTWARE;