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This adds a SurfaceLayerForMediaStreams feature that is enabled by
default. The expectation is that the Meet team will use this to
evaluate the CPU usage and other performance characteristics.

To disable surface layer for media streams you should launch Chrome
with --disable-features=SurfaceLayerForMediaStreams

R=liberato

Bug: 1157671
Test: http://apprtc.appspot.com/
Change-Id: I89cca10f8a81939d4a742be4352dcd2075edc6c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586513
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836309}