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This CL adds a SoftwareOutputDevice implementation for Windows software
compositing to a layered window. The layered window API is required to
support transparency when DWM is disabled. Calling UpdateLayeredWindow()
is also blocked by the GPU sandbox.

To work around this SoftwareOutputDeviceWinProxy in the GPU process
draws into a shared memory buffer and sends an IPC to the browser
process. LayeredWindowUpdaterImpl receives the IPC and calls
UpdateLayeredWindow().

Having the browser process draw into an HWND with pixels produced in the
GPU process shouldn't be a security issue. The GPU process can already
draw into the HWND via other APIs. Drawing is clipped to the HWND size.

It's not ideal having to send an IPC from the GPU to browser process for
each draw. However, layered windows are the only way for an HWND to
guarantee it supports transparency on Windows 7. Having a child window
in the GPU process is not possible as layered windows are incompatible
with child windows for Windows 7.

HWNDs with transparent backgrounds are used infrequently and don't
update quickly so this should hopefully be performant enough. This code
is also only used for Windows 7.

Bug: 826633
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I3b2d749b75aed7789b19b3e8dfd7d0371ef6fe73
Reviewed-on: https://chromium-review.googlesource.com/1042283
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558462}