Implement H.264 packetization mode 0.

This approach extends the H.264 specific information with
a packetization mode enum.

Status: Parameter is in code. No way to set it yet.

Rebase of CL  2009213002

BUG=600254

Review-Url: https://codereview.webrtc.org/2337453002
Cr-Commit-Position: refs/heads/master@{#15032}

webrtc/modules/video_coding/include/video_codec_interface.h

@@ -77,7 +77,9 @@ struct CodecSpecificInfoGeneric {
   uint8_t simulcast_idx;
 };
 
-struct CodecSpecificInfoH264 {};
+struct CodecSpecificInfoH264 {
+  H264PacketizationMode packetization_mode;
+};
 
 union CodecSpecificInfoUnion {
   CodecSpecificInfoGeneric generic;