Revert of Rename RTPVideoHeader.isFirstPacket to .is_first_packet_in_frame.

webrtc/modules/video_coding/nack_module.cc

Index: webrtc/modules/video_coding/nack_module.cc
diff --git a/webrtc/modules/video_coding/nack_module.cc b/webrtc/modules/video_coding/nack_module.cc
index d74f08952617da38ad7a7d75b7f87067021816a5..43244321ea0ed5d883a5aadf9e7fa8ed070995b8 100644
--- a/webrtc/modules/video_coding/nack_module.cc
+++ b/webrtc/modules/video_coding/nack_module.cc
@@ -66,8 +66,7 @@
   //                 now set it to true, which will cause the reordering
   //                 statistics to never be updated.
   bool is_retransmitted = true;
-  bool is_keyframe =
-      packet.is_first_packet_in_frame && packet.frameType == kVideoFrameKey;
+  bool is_keyframe = packet.isFirstPacket && packet.frameType == kVideoFrameKey;
 
   if (!initialized_) {
     newest_seq_num_ = seq_num;