Reland of Rename RTPVideoHeader.isFirstPacket to .is_first_packet_in_frame.

Add RTC_DEPRACATed anonymous unions to not break downstream projects.

Orignal issue's description:
> commit 0ad21111fcc57a7e978edba3c4263f0062d7f9ff
> Author: danilchap <danilchap@webrtc.org>
> Date:   Mon Dec 19 09:36:33 2016 -0800
>
>     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}
>

BUG=None

Review-Url: https://codereview.webrtc.org/2614503002
Cr-Commit-Position: refs/heads/master@{#15987}
diff --git a/webrtc/modules/video_coding/video_receiver_unittest.cc b/webrtc/modules/video_coding/video_receiver_unittest.cc
index f7241ca..9cac45b 100644
--- a/webrtc/modules/video_coding/video_receiver_unittest.cc
+++ b/webrtc/modules/video_coding/video_receiver_unittest.cc
@@ -123,14 +123,14 @@
   header.type.Video.codec = kRtpVideoVp8;
   // Insert one video frame to get one frame decoded.
   header.frameType = kVideoFrameKey;
-  header.type.Video.isFirstPacket = true;
+  header.type.Video.is_first_packet_in_frame = true;
   header.header.markerBit = true;
   InsertAndVerifyDecodableFrame(payload, kFrameSize, &header);
   clock_.AdvanceTimeMilliseconds(33);
   header.header.timestamp += 3000;
 
   header.frameType = kEmptyFrame;
-  header.type.Video.isFirstPacket = false;
+  header.type.Video.is_first_packet_in_frame = false;
   header.header.markerBit = false;
   // Insert padding frames.
   for (int i = 0; i < 10; ++i) {
@@ -166,7 +166,7 @@
   WebRtcRTPHeader header;
   memset(&header, 0, sizeof(header));
   header.frameType = kEmptyFrame;
-  header.type.Video.isFirstPacket = false;
+  header.type.Video.is_first_packet_in_frame = false;
   header.header.markerBit = false;
   header.header.paddingLength = kPaddingSize;
   header.header.payloadType = kUnusedPayloadType;
@@ -182,7 +182,7 @@
         header.frameType = kVideoFrameKey;
       else
         header.frameType = kVideoFrameDelta;
-      header.type.Video.isFirstPacket = true;
+      header.type.Video.is_first_packet_in_frame = true;
       header.header.markerBit = true;
       InsertAndVerifyDecodableFrame(payload, kFrameSize, &header);
       clock_.AdvanceTimeMilliseconds(33);
@@ -191,7 +191,7 @@
 
     // Insert 2 padding only frames.
     header.frameType = kEmptyFrame;
-    header.type.Video.isFirstPacket = false;
+    header.type.Video.is_first_packet_in_frame = false;
     header.header.markerBit = false;
     for (int j = 0; j < 2; ++j) {
       // InsertAndVerifyPaddingFrame(payload, &header);