Move VideoFrame and related declarations to webrtc/api/video.

Moves webrtc/common_video/rotation.h and parts of
webrtc/common_video/include/video_frame_buffer.h and
webrtc/video_frame.h, and adds to a new GN target api:video_frame_api.

BUG=webrtc:5880

Review-Url: https://codereview.webrtc.org/2517173004
Cr-Commit-Position: refs/heads/master@{#15993}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
index 4b2b754..5805e30 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
@@ -12,7 +12,7 @@
 
 #include <memory>
 
-#include "webrtc/common_video/rotation.h"
+#include "webrtc/api/video/video_rotation.h"
 
 @implementation RTCVideoFrame {
   rtc::scoped_refptr<webrtc::VideoFrameBuffer> _videoBuffer;