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This test was expecting the main thread copy request to happen before a
new draw happens on the compositor thread which is racey. Instead do
this comparison on the main thread by posting and trying to end the test
after the draw happens but checking for the copy request result there
on the main thread instead of on the impl thread before posting.

R=vmpstr, weiliangc
BUG=439649

Review URL: https://codereview.chromium.org/888663003

Cr-Commit-Position: refs/heads/master@{#313797}