Revert of Rename RTPVideoHeader.isFirstPacket to .is_first_packet_in_frame.

webrtc/video/video_send_stream_tests.cc

Index: webrtc/video/video_send_stream_tests.cc
diff --git a/webrtc/video/video_send_stream_tests.cc b/webrtc/video/video_send_stream_tests.cc
index e2b256d403e869667bbed1b21047f627cfddfae9..7a11ae1243b190aac712275e379c9302968b0ca7 100644
--- a/webrtc/video/video_send_stream_tests.cc
+++ b/webrtc/video/video_send_stream_tests.cc
@@ -2941,7 +2941,7 @@
 
     bool new_frame = packets_sent_ == 0 ||
                      IsNewerTimestamp(header.timestamp, last_header_.timestamp);
-    EXPECT_EQ(new_frame, video.is_first_packet_in_frame);
+    EXPECT_EQ(new_frame, video.isFirstPacket);
     if (!new_frame) {
       EXPECT_FALSE(last_header_.markerBit);
       EXPECT_EQ(last_header_.timestamp, header.timestamp);