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/decoding_state_unittest.cc b/webrtc/modules/video_coding/decoding_state_unittest.cc
index 1edd1e3..c147354 100644
--- a/webrtc/modules/video_coding/decoding_state_unittest.cc
+++ b/webrtc/modules/video_coding/decoding_state_unittest.cc
@@ -32,7 +32,7 @@
   VCMFrameBuffer frame;
   VCMFrameBuffer frame_key;
   VCMPacket packet;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.timestamp = 1;
   packet.seqNum = 0xffff;
   packet.frameType = kVideoFrameDelta;
@@ -52,7 +52,7 @@
   frame.Reset();
   packet.frameType = kVideoFrameDelta;
   // Use pictureId
-  packet.is_first_packet_in_frame = false;
+  packet.isFirstPacket = false;
   packet.video_header.codecHeader.VP8.pictureId = 0x0002;
   EXPECT_LE(0, frame.InsertPacket(packet, 0, kNoErrors, frame_data));
   EXPECT_FALSE(dec_state.ContinuousFrame(&frame));
@@ -260,7 +260,7 @@
   // A key frame is always a base layer.
   frame.Reset();
   packet.frameType = kVideoFrameKey;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.timestamp = 5;
   packet.seqNum = 5;
   packet.video_header.codecHeader.VP8.tl0PicIdx = 2;
@@ -284,7 +284,7 @@
   EXPECT_TRUE(dec_state.full_sync());
   frame.Reset();
   packet.frameType = kVideoFrameDelta;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.timestamp = 8;
   packet.seqNum = 8;
   packet.video_header.codecHeader.VP8.tl0PicIdx = 4;
@@ -299,7 +299,7 @@
   // Insert a non-ref frame - should update sync value.
   frame.Reset();
   packet.frameType = kVideoFrameDelta;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.timestamp = 9;
   packet.seqNum = 9;
   packet.video_header.codecHeader.VP8.tl0PicIdx = 4;
@@ -319,7 +319,7 @@
   frame.Reset();
   dec_state.Reset();
   packet.frameType = kVideoFrameDelta;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.markerBit = 1;
   packet.timestamp = 0;
   packet.seqNum = 0;
@@ -333,7 +333,7 @@
   // Layer 2 - 2 packets (insert one, lose one).
   frame.Reset();
   packet.frameType = kVideoFrameDelta;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.markerBit = 0;
   packet.timestamp = 1;
   packet.seqNum = 1;
@@ -346,7 +346,7 @@
   // Layer 1
   frame.Reset();
   packet.frameType = kVideoFrameDelta;
-  packet.is_first_packet_in_frame = 1;
+  packet.isFirstPacket = 1;
   packet.markerBit = 1;
   packet.timestamp = 2;
   packet.seqNum = 3;
@@ -450,7 +450,7 @@
   VCMDecodingState dec_state;
   VCMFrameBuffer frame;
   VCMPacket packet;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.timestamp = 1;
   packet.seqNum = 0xffff;
   uint8_t data[] = "I need a data pointer for this test!";
@@ -493,7 +493,7 @@
   VCMDecodingState dec_state;
   VCMFrameBuffer frame;
   VCMPacket packet;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.timestamp = 1;
   packet.seqNum = 0xffff;
   uint8_t data[] = "I need a data pointer for this test!";
@@ -548,7 +548,7 @@
   VCMDecodingState dec_state;
   VCMFrameBuffer frame;
   VCMPacket packet;
-  packet.is_first_packet_in_frame = true;
+  packet.isFirstPacket = true;
   packet.timestamp = 1;
   packet.seqNum = 0xffff;
   uint8_t data[] = "I need a data pointer for this test!";