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As part of the UseRealBuffersForPageFlipTest experiment,
OverlayProcessorOzone now uses the primary plane's buffer directly
for page flip tests [1]. This buffer is obtained via
OutputSurface::GetOverlayMailbox [2] which is unfortunately only
implemented for GL-based OutputSurfaces.

With SkiaRenderer enabled, OverlayProcessorOzone can't extract the
primary plane mailbox and skips promoting overlays at all [3]. This
CL implements GetOverlayMailbox for SkiaOutputSurface by threading
the corresponding mailbox from the GPU main thread back to the
compositor thread via the SwapBuffers callback.

Note that since GetOverlayMailbox corresponds to the *last* submitted
buffer, we need to skip the primary plane in the event of a reshape
because the page flip test result is meaningless.

[1] https://source.chromium.org/chromium/chromium/src/+/master:components/viz/service/display/overlay_processor_ozone.cc;l=139;drc=a44fcb321ea308b8f8d2227190a99c6cca2b79e6
[2] https://source.chromium.org/chromium/chromium/src/+/master:components/viz/service/display/direct_renderer.cc;l=356;drc=a44fcb321ea308b8f8d2227190a99c6cca2b79e6
[3] https://source.chromium.org/chromium/chromium/src/+/master:components/viz/service/display/overlay_processor_ozone.cc;l=210;drc=a44fcb321ea308b8f8d2227190a99c6cca2b79e6

Change-Id: I88d10746ede9bdc8afa565e8a42aef27d5572e53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533416
Commit-Queue: Brian Ho <hob@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Kramer Ge <fangzhoug@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828865}