Add support for VP9 packetization/depacketization.

RTP payload format for VP9:
https://www.ietf.org/id/draft-uberti-payload-vp9-01.txt

BUG=webrtc:4148, webrtc:4168, chromium:500602
TBR=mflodman

Review URL: https://codereview.webrtc.org/1232023006

Cr-Commit-Position: refs/heads/master@{#9649}

webrtc/modules/video_coding/main/source/packet.cc

@@ -118,6 +118,18 @@ void VCMPacket::CopyCodecSpecifics(const RTPVideoHeader& videoHeader) {
 
       codec = kVideoCodecVP8;
       return;
+    case kRtpVideoVp9:
+      if (isFirstPacket && markerBit)
+        completeNALU = kNaluComplete;
+      else if (isFirstPacket)
+        completeNALU = kNaluStart;
+      else if (markerBit)
+        completeNALU = kNaluEnd;
+      else
+        completeNALU = kNaluIncomplete;
+
+      codec = kVideoCodecVP9;
+      return;
     case kRtpVideoH264:
       isFirstPacket = videoHeader.isFirstPacket;
       if (isFirstPacket)