Ignore prefix NAL unit for slice parsing.

For temporal scalability, prefix NALU contains layer information, but
should not be parsed as a slice.

Bug: webrtc:12991
Change-Id: Ic1e7d41f568310390a743d4ace016aa7d57a4864
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226501
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34523}

common_video/h264/h264_bitstream_parser.cc

@@ -288,6 +288,7 @@ void H264BitstreamParser::ParseSlice(const uint8_t* slice, size_t length) {
     }
     case H264::NaluType::kAud:
     case H264::NaluType::kSei:
+    case H264::NaluType::kPrefix:
       break;  // Ignore these nalus, as we don't care about their contents.
     default:
       Result res = ParseNonParameterSetNalu(slice, length, nalu_type);

common_video/h264/h264_common.h

@@ -42,6 +42,7 @@ enum NaluType : uint8_t {
   kEndOfSequence = 10,
   kEndOfStream = 11,
   kFiller = 12,
+  kPrefix = 14,
   kStapA = 24,
   kFuA = 28
 };