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/nack_module_unittest.cc b/webrtc/modules/video_coding/nack_module_unittest.cc
index bb581fe..fb7d095 100644
--- a/webrtc/modules/video_coding/nack_module_unittest.cc
+++ b/webrtc/modules/video_coding/nack_module_unittest.cc
@@ -72,7 +72,7 @@
 
   sent_nacks_.clear();
   packet.frameType = kVideoFrameKey;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.seqNum = 2;
   nack_module_.OnReceivedPacket(packet);
   EXPECT_EQ(0u, sent_nacks_.size());
@@ -239,11 +239,11 @@
   packet.seqNum = 0;
   nack_module_.OnReceivedPacket(packet);
   packet.seqNum = 1;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.frameType = kVideoFrameKey;
   nack_module_.OnReceivedPacket(packet);
   packet.seqNum = 1001;
-  packet.is_first_packet_in_frame = false;
+  packet.isFirstPacket = false;
   packet.frameType = kVideoFrameKey;
   nack_module_.OnReceivedPacket(packet);
   EXPECT_EQ(999u, sent_nacks_.size());