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This relates to the following two classes used when capturing
content on macOS:
- AVCaptureDevice which does the actual capturing from the device
  into IOSurfaces
- AVCaptureVideoDataOutput takes those IOSurfaces and wrangles
  them into CMSampleBuffers that are handed to us

Bug 1: Not telling AVCaptureDevice our preferences
  When Chrome wants a specific (width, height, framerate)
  combination, we request that, along with a pixel format,
  to AVCaptureVideoDataOutput.

  While we indicate our preferences to our
  AVCaptureVideoDataOutput, we do not indicate them to our
  AVCaptureDevice.

  As a result, the AVCaptureDevice will often have a completely
  different format that it's using, and the aforementioned
  "wrangling" will involve a format conversion and maybe even
  some downsampling(!). In bad cases, this can burn 10% of a
  CPU core on its own (fortunately bad cases are rare).

Bug 2: Using an unsupported pixel format
  We currently prefer "422YpCbCr8" aka "UYVY" aka "2vuy", and
  request it regardless of whether or not it is supported by
  the capture. Most devices don't actually support it, and so
  the wrangling starts up again.

The fix for these bugs is to trawl through the list of supported
pixel formats to find the one that best matches the requested
format, and then specify this to our AVCaptureDevice when we
start capture.

Do this by adding a FindBestCaptureFormat to do the trawling.

Also don't prefer the "422YpCbCr8" aka "UYVY" aka "2vuy" format
above supported formats.

The consequence of this is that many Facetime cameras will now
be outputting "422YpCbCr8_yuvs" aka "YUY2" aka "yuvs", and will
hopefully use less CPU.

When we do get an exact match between input and output, we
also gain access to the IOSurface that the camera's data
is being thrown into. This could allow a zero copy path all
the way through to the compositor. Eventually.

Bug: 1124884
Change-Id: I33b4b62e152ba9f3b2be04dc3a5f3084d6ee9438
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391767
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804584}