Reason for revert:
Keep header file for backwards compatibility
Original issue's description:
> Revert of Move CoreVideoFrameBuffer from webrtc/common_video/ to webrtc/sdk/objc/ (patchset #2 id:60001 of https://codereview.webrtc.org/2851563003/ )
>
> Reason for revert:
> Breaks downstream targets.
>
> Original issue's description:
> > Move CoreVideoFrameBuffer from webrtc/common_video/ to webrtc/sdk/objc/
> >
> > CoreVideoFrameBuffer is Mac/iPhone specific and should be moved into
> > the webrtc/sdk/objc/ folder.
> >
> > BUG=None
> >
> > Review-Url: https://codereview.webrtc.org/2851563003
> > Cr-Commit-Position: refs/heads/master@{#17998}
> > Committed: d41631aa27
>
> TBR=kthelgason@webrtc.org,magjed@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=None
>
> Review-Url: https://codereview.webrtc.org/2862663003
> Cr-Commit-Position: refs/heads/master@{#18004}
> Committed: c34e730896TBR=kthelgason@webrtc.org,ehmaldonado@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=None
Review-Url: https://codereview.webrtc.org/2860033002
Cr-Commit-Position: refs/heads/master@{#18011}
173658968e