Cleanup: Remove i420_video_frame.h header.
Cleanup: Remove i420_video_frame.h header.

It is just a pass through to webrtc/video_frame.h. Updated the callers
to include webrtc/video_frame.h instead and removed i420_video_frame.h.

This should fix pbos' TODO in i420_video_frame.h.

Tested on Linux with the following command lines:

$ rm -rf out/
$ ./webrtc/build/gyp_webrtc
$ ninja -C out/Debug

BUG=None
TEST=see above
R=magjed@webrtc.org, pbos@webrtc.org, tommi@webrtc.org
TBR=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46819004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8973}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9bfe3daf7349b62647997ced9389baa8ab043afe

dcpang/fatline-libjingle,sippet/talk,sippet/talk,sippet/talk,sippet/talk,dcpang/fatline-libjingle,dcpang/fatline-libjingle,dcpang/fatline-libjingle,dcpang/fatline-libjingle,sippet/talk