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/test/frame_generator_capturer.h b/webrtc/test/frame_generator_capturer.h
index 86df689..aff906d 100644
--- a/webrtc/test/frame_generator_capturer.h
+++ b/webrtc/test/frame_generator_capturer.h
@@ -45,6 +45,7 @@
 
   void Start() override;
   void Stop() override;
+  void ForceFrame();
 
   int64_t first_frame_capture_time() const { return first_frame_capture_time_; }