[webcodecs] Ignore access unit delimiter from H.264 conversion.

Bug: None
Change-Id: I99bdb4af8f9e4e1958a2e148fddded0692345e06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508962Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Chunbo Hua <chunbo.hua@intel.com>
Cr-Commit-Position: refs/heads/master@{#822961}

media/formats/mp4/h264_annex_b_to_avc_bitstream_converter.cc

@@ -58,6 +58,9 @@ Status H264AnnexBToAvcBitstreamConverter::ConvertChunk(
                     "Failed to parse H.264 stream");
 
     switch (nalu.nal_unit_type) {
+      case H264NALU::kAUD: {
+        break;
+      }
       case H264NALU::kSPS: {
         int sps_id = -1;
         result = parser_.ParseSPS(&sps_id);