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/source/generic_decoder.cc

@@ -46,8 +46,7 @@ VCMReceiveCallback* VCMDecodedFrameCallback::UserReceiveCallback()
     return _receiveCallback;
 }
 
-int32_t VCMDecodedFrameCallback::Decoded(I420VideoFrame& decodedImage)
-{
+int32_t VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage) {
     // TODO(holmer): We should improve this so that we can handle multiple
     // callbacks from one call to Decode().
     VCMFrameInformation* frameInfo;