Add SEI to the list of packets counted as keyframe.

It's generated by some encoders between SPS/PPS and an IDR frame, so we should treat it like sps/pps.

BUG=

Review URL: https://codereview.webrtc.org/1664733002

Cr-Commit-Position: refs/heads/master@{#11470}

webrtc/modules/rtp_rtcp/source/rtp_format_h264.cc

@@ -101,6 +101,7 @@ bool ParseSingleNalu(RtpDepacketizer::ParsedPayload* parsed_payload,
   switch (nal_type) {
     case kSps:
     case kPps:
+    case kSei:
     case kIdr:
       parsed_payload->frame_type = kVideoFrameKey;
       break;