Fix broken CVO header extension

Adds end to end unit tests for CVO.

BUG=webrtc:5621

Review URL: https://codereview.webrtc.org/1811373002

Cr-Commit-Position: refs/heads/master@{#12063}
diff --git a/webrtc/media/base/fakevideorenderer.h b/webrtc/media/base/fakevideorenderer.h
index 05e285b..5cc1a59 100644
--- a/webrtc/media/base/fakevideorenderer.h
+++ b/webrtc/media/base/fakevideorenderer.h
@@ -59,7 +59,7 @@
     rtc::CritScope cs(&crit_);
     return height_;
   }
-  int rotation() const {
+  webrtc::VideoRotation rotation() const {
     rtc::CritScope cs(&crit_);
     return rotation_;
   }