Add H.264 packetization.

This also includes:
- Creating new packetizer and depacketizer interfaces.
- Moved VP8 packetization was H264 packetization and depacketization to these interfaces. This is a work in progress and should be continued to get this 100% generic. This also required changing the return type for RtpFormatVp8::NextPacket(), which now returns bool instead of the index of the first partition.
- Created a Create() factory method for packetizers and depacketizers.

R=niklas.enbom@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21009004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6804 4adac7df-926f-26a2-2b94-8c16560cd09d

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

@@ -38,6 +38,9 @@ void CopyCodecSpecific(const CodecSpecificInfo* info, RTPVideoHeader** rtp) {
       (*rtp)->simulcastIdx = info->codecSpecific.VP8.simulcastIdx;
       return;
     }
+    case kVideoCodecH264:
+      (*rtp)->codec = kRtpVideoH264;
+      return;
     case kVideoCodecGeneric:
       (*rtp)->codec = kRtpVideoGeneric;
       (*rtp)->simulcastIdx = info->codecSpecific.generic.simulcast_idx;