Rename I420VideoFrame to VideoFrame.

This is a mechanical change since it affects so many
files.
I420VideoFrame -> VideoFrame
and reformatted.

Rationale: in the next CL I420VideoFrame will
get an indication of Pixel Format (I420 for
starters) and of storage type: usually
UNOWNED, could be SHMEM, and in the near
future will be possibly TEXTURE. See
https://codereview.chromium.org/1154153003
for the change that happened in Cr.

BUG=4730, chromium:440843
R=jiayl@webrtc.org, niklas.enbom@webrtc.org, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/52629004

Cr-Commit-Position: refs/heads/master@{#9339}

webrtc/modules/video_coding/main/test/test_util.h

@@ -57,7 +57,7 @@ class FileOutputFrameReceiver : public webrtc::VCMReceiveCallback {
   virtual ~FileOutputFrameReceiver();
 
   // VCMReceiveCallback
-  virtual int32_t FrameToRender(webrtc::I420VideoFrame& video_frame);
+  virtual int32_t FrameToRender(webrtc::VideoFrame& video_frame);
 
  private:
   std::string out_filename_;