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}
diff --git a/webrtc/modules/video_coding/frame_buffer.cc b/webrtc/modules/video_coding/frame_buffer.cc
index bb8ec93..e3b46de 100644
--- a/webrtc/modules/video_coding/frame_buffer.cc
+++ b/webrtc/modules/video_coding/frame_buffer.cc
@@ -161,7 +161,7 @@
     _rotation_set = true;
   }
 
-  if (packet.isFirstPacket) {
+  if (packet.is_first_packet_in_frame) {
     playout_delay_ = packet.video_header.playout_delay;
   }