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/packet_buffer.cc b/webrtc/modules/video_coding/packet_buffer.cc
index 03aeb5a..bbf488c 100644
--- a/webrtc/modules/video_coding/packet_buffer.cc
+++ b/webrtc/modules/video_coding/packet_buffer.cc
@@ -103,7 +103,7 @@
     if (AheadOf(seq_num, last_seq_num_))
       last_seq_num_ = seq_num;
 
-    sequence_buffer_[index].frame_begin = packet->isFirstPacket;
+    sequence_buffer_[index].frame_begin = packet->is_first_packet_in_frame;
     sequence_buffer_[index].frame_end = packet->markerBit;
     sequence_buffer_[index].seq_num = packet->seqNum;
     sequence_buffer_[index].continuous = false;