media/gpu/h264_decoder: work around seeking bug with Youtube app

The Youtube Android app will sometimes send an invalid frame number
after seeking, which triggers an error with the H264Decoder. Until this
is fixed, this workaround prevents decoding from failing every second
seek.

BUG=b:146914440
BUG=b:129119729
TEST=Can seek without issue or artefacts in the Android Youtube app on
Krane.

Change-Id: I8b6ca5f4c53a67e934c918023ab987e7a0009ffb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2000258
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732728}

media/gpu/h264_decoder.cc

@@ -1127,7 +1127,12 @@ bool H264Decoder::HandleFrameNumGap(int frame_num) {
 
   if (!sps->gaps_in_frame_num_value_allowed_flag) {
     DVLOG(1) << "Invalid frame_num: " << frame_num;
-    return false;
+    // TODO(b:129119729, b:146914440): Youtube android app sometimes sends an
+    // invalid frame number after a seek. The sequence goes like:
+    // Seek, SPS, PPS, IDR-frame, non-IDR, ... non-IDR with invalid number.
+    // The only way to work around this reliably is to ignore this error.
+    // Video playback is not affected, no artefacts are visible.
+    // return false;
   }
 
   DVLOG(2) << "Handling frame_num gap: " << prev_ref_frame_num_ << "->"