Add support for transport wide sequence numbers

Also refactor packet router to use a map rather than iterate over all
rtp modules for each packet sent.

BUG=webrtc:4311

Review URL: https://codereview.webrtc.org/1247293002

Cr-Commit-Position: refs/heads/master@{#9670}
diff --git a/webrtc/modules/bitrate_controller/include/bitrate_controller.h b/webrtc/modules/bitrate_controller/include/bitrate_controller.h
index 7303d06..bb53288 100644
--- a/webrtc/modules/bitrate_controller/include/bitrate_controller.h
+++ b/webrtc/modules/bitrate_controller/include/bitrate_controller.h
@@ -23,6 +23,7 @@
 namespace webrtc {
 
 class CriticalSectionWrapper;
+struct PacketInfo;
 
 class BitrateObserver {
   // Observer class for bitrate changes announced due to change in bandwidth