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This CL implements a WebRTC Desktop Capturer that captures from
the Aura Shell root window.

The capturer is used by the It2MeHost on Chrome OS.
|DesktopCaptureDeviceAura| cannot be re-used in our scenario
because is timer-driven as opposed to caller driven, which is
required by WebRTC.

The current implementation uses the layer API

desktop_window_->layer()->RequestCopyOfOutput(request.Pass())

to request the layer and its subtree to be rendered to a
|SkiaBitmap|.  It then copies the pixels to a WebRTC |DesktopFrame|.

BUG=411530

Committed: https://crrev.com/278b065b519617b1e9231bd53e6502706d20787c
Cr-Commit-Position: refs/heads/master@{#295187}

Review URL: https://codereview.chromium.org/543243003

Cr-Commit-Position: refs/heads/master@{#295214}