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On MacOS, GrabViewSnapshot() call would block for the duration of
display/GPU sleep which leads to protocol client to wait and tests
to time out when power saving is engaged. This prevents power saving
while there are pending screenshot requests.

BUG=599776

Review-Url: https://codereview.chromium.org/2044783002
Cr-Commit-Position: refs/heads/master@{#398245}