Rename RTPVideoHeader.isFirstPacket to .is_first_packet_in_frame.

Name should represent the actual meaning.

BUG=None

Review-Url: https://codereview.webrtc.org/2574943003
Cr-Commit-Position: refs/heads/master@{#15684}

webrtc/modules/video_coding/nack_module.cc

@@ -66,7 +66,8 @@ int NackModule::OnReceivedPacket(const VCMPacket& packet) {
   //                 now set it to true, which will cause the reordering
   //                 statistics to never be updated.
   bool is_retransmitted = true;
-  bool is_keyframe = packet.isFirstPacket && packet.frameType == kVideoFrameKey;
+  bool is_keyframe =
+      packet.is_first_packet_in_frame && packet.frameType == kVideoFrameKey;
 
   if (!initialized_) {
     newest_seq_num_ = seq_num;