Revert of Rename RTPVideoHeader.isFirstPacket to .is_first_packet_in_frame. (patchset #1 id:1 of https://codereview.webrtc.org/2574943003/ )

Reason for revert:
breaks downstream project.

Can you make this change in a compatible way using anonymous union:
union {
  bool is_first_packet_in_frame;
  RTC_DEPRECATED bool isFirstPacket;
};
(unfortunetly this this treak breaks braced initialization in rtp_rtcp_impl_unittest.cc,
so that should be rewritting in a more classic way)

Original issue's description:
> 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}
> Committed: efde908380

TBR=stefan@webrtc.org,sprang@webrtc.org,johan@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=None

Review-Url: https://codereview.webrtc.org/2589783003
Cr-Commit-Position: refs/heads/master@{#15686}

webrtc/modules/video_coding/nack_module.cc

@@ -66,8 +66,7 @@ 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.is_first_packet_in_frame && packet.frameType == kVideoFrameKey;
+  bool is_keyframe = packet.isFirstPacket && packet.frameType == kVideoFrameKey;
 
   if (!initialized_) {
     newest_seq_num_ = seq_num;