To ensure this change won't break Chromium, this is the first change, to add a
new CaptureFrame() function, and let Capture(DesktopRegion) and CaptureFrame()
call each other. So both a legacy consumer or a legacy implementation won't be
broken.
BUG=https://bugs.chromium.org/p/webrtc/issues/detail?id=6513
Review-Url: https://codereview.webrtc.org/2409833002
Cr-Commit-Position: refs/heads/master@{#14635}