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This adds a zero-copy desktop capture mechanism to macOS. The frames
are captured as NV12 IOSurfaces, packaged in IOSurfaces, and passed
directly to the consumers (e.g, compositor, encoders, etc).

This implementation is a trimmed-down version of the existing system,
which can be found in the DesktopCaptureDevice class and the WebRTC
class ScreenCapturerMac. The CGDisplayStream code is identical to
the WebRTC code except for the following changes:
- It requests NV12 as the capture format
- It specifies the requested capture resolution (rather than capturing
  at retina resolution and downsampling in software)
- It specifies kCGDisplayStreamMinimumFrameTime and
  kCGDisplayStreamColorSpace.
Otherwise, all calls are identical.

Bug: 1164736
Change-Id: I962911aa9b1863d730c333866f5644bbbd2b7164
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616802
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Markus Handell <handellm@google.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843989}