Init alpha_codec_config_ in VpxEncoder

content/renderer/media_recorder/video_track_recorder.cc

Index: content/renderer/media_recorder/video_track_recorder.cc
diff --git a/content/renderer/media_recorder/video_track_recorder.cc b/content/renderer/media_recorder/video_track_recorder.cc
index d77d300d8da957f3b0675927fff963acca09c093..02e77145479dc40ae66452507190fb9a0cbe7296 100644
--- a/content/renderer/media_recorder/video_track_recorder.cc
+++ b/content/renderer/media_recorder/video_track_recorder.cc
@@ -780,6 +780,7 @@ VpxEncoder::VpxEncoder(
     : Encoder(on_encoded_video_callback, bits_per_second),
       use_vp9_(use_vp9) {
   codec_config_.g_timebase.den = 0;  // Not initialized.
+  alpha_codec_config_.g_timebase.den = 0;  // Not initialized.
   DCHECK(encoding_thread_->IsRunning());
 }