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Changes the interface to use SkBitmap, which is passed over Mojo IPC
using BigBuffer (shared memory). This is then wrapped using a special
subclass of webrtc::DesktopFrame (one less copy).

There are still more copies than would be desirable here. We have the
initial copy created by Ash. That is translated into a BigBuffer for
transport over Mojo. That is then copied back to a SkBitmap. Avoiding
those copies and even reusing the transport buffer are an exercise for
a later CL.

The interface design also unifies screen and window capture in a way
that is consistent with webrtc::DesktopCapturer. This also enables a
path to adding support for multiple screens in a future CL.

There's a good deal of complexity in this CL to handle version skew.
QueryVersion doesn't work synchronously, so a nested event loop is
used on the background thread where DesktopCapturerLacros runs. That
is safe, since this is a dedicated thread, but not ideal.

The deprecated methods are re-implemented in terms of the newer
interface.

Bug: 1094460
Change-Id: I2b2f86df08612b79faae7da547ff1242c05ac467
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443404Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814382}