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: https://chromium.googlesource.com/external/webrtc/+/efde90838055f44ca05863ba020ca02c88b6d14c

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}
diff --git a/webrtc/modules/video_coding/packet.cc b/webrtc/modules/video_coding/packet.cc
index 4d5ba7c..643a10a 100644
--- a/webrtc/modules/video_coding/packet.cc
+++ b/webrtc/modules/video_coding/packet.cc
@@ -27,7 +27,7 @@
       timesNacked(-1),
       frameType(kEmptyFrame),
       codec(kVideoCodecUnknown),
-      is_first_packet_in_frame(false),
+      isFirstPacket(false),
       completeNALU(kNaluUnset),
       insertStartCode(false),
       width(0),
@@ -49,7 +49,7 @@
       timesNacked(-1),
       frameType(rtpHeader.frameType),
       codec(kVideoCodecUnknown),
-      is_first_packet_in_frame(rtpHeader.type.Video.is_first_packet_in_frame),
+      isFirstPacket(rtpHeader.type.Video.isFirstPacket),
       completeNALU(kNaluComplete),
       insertStartCode(false),
       width(rtpHeader.type.Video.width),
@@ -61,7 +61,7 @@
     video_header.rotation = rtpHeader.type.Video.rotation;
   }
   // Playout decisions are made entirely based on first packet in a frame.
-  if (is_first_packet_in_frame) {
+  if (isFirstPacket) {
     video_header.playout_delay = rtpHeader.type.Video.playout_delay;
   } else {
     video_header.playout_delay = {-1, -1};
@@ -79,7 +79,7 @@
   timesNacked = -1;
   frameType = kEmptyFrame;
   codec = kVideoCodecUnknown;
-  is_first_packet_in_frame = false;
+  isFirstPacket = false;
   completeNALU = kNaluUnset;
   insertStartCode = false;
   width = 0;
@@ -93,9 +93,9 @@
       // Handle all packets within a frame as depending on the previous packet
       // TODO(holmer): This should be changed to make fragments independent
       // when the VP8 RTP receiver supports fragments.
-      if (is_first_packet_in_frame && markerBit)
+      if (isFirstPacket && markerBit)
         completeNALU = kNaluComplete;
-      else if (is_first_packet_in_frame)
+      else if (isFirstPacket)
         completeNALU = kNaluStart;
       else if (markerBit)
         completeNALU = kNaluEnd;
@@ -105,9 +105,9 @@
       codec = kVideoCodecVP8;
       return;
     case kRtpVideoVp9:
-      if (is_first_packet_in_frame && markerBit)
+      if (isFirstPacket && markerBit)
         completeNALU = kNaluComplete;
-      else if (is_first_packet_in_frame)
+      else if (isFirstPacket)
         completeNALU = kNaluStart;
       else if (markerBit)
         completeNALU = kNaluEnd;
@@ -117,13 +117,13 @@
       codec = kVideoCodecVP9;
       return;
     case kRtpVideoH264:
-      is_first_packet_in_frame = videoHeader.is_first_packet_in_frame;
-      if (is_first_packet_in_frame)
+      isFirstPacket = videoHeader.isFirstPacket;
+      if (isFirstPacket)
         insertStartCode = true;
 
-      if (is_first_packet_in_frame && markerBit) {
+      if (isFirstPacket && markerBit) {
         completeNALU = kNaluComplete;
-      } else if (is_first_packet_in_frame) {
+      } else if (isFirstPacket) {
         completeNALU = kNaluStart;
       } else if (markerBit) {
         completeNALU = kNaluEnd;