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A number of layout tests requests a testRunner.capturePixelsAsyncThen().
When not using the display-compositor for pixel-dumps, this triggers a
complete composition (i.e. this triggers a commit, raster, frame
submission, and processing in the display compositor). The tests that
use the testRunner.capturePixelsAsyncThen(callback) API expects these
steps to be completed before the callback is run. However, when using the
display-compositor for pixel-dumps, it immediately triggers the callback
without doing any of these steps. This caused a bunch of tests to fail,
or become flaky, when --enable-display-compositor-pixel-dump became the
default.

This patch fixes the flakiness, by making sure the callback is run only
after all the required steps have been performed. This is done by forcing
a redraw, and requesting a presentation-timestamp from viz. This does not
actually fix the readback itself, i.e. the pixels returned to the
callback is still invalid. However, this does fix the timing of running
the callback. That is sufficient to fix a number of the existing flaky
tests.

Bug: 891427
Change-Id: Ic09db1c3a0a410b2d5687999d7144260df7ea272
Reviewed-on: https://chromium-review.googlesource.com/c/1322364Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607906}