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isAcceleratedCompositingActive is a racy check asking if the page has
had a compositing update with a non-null root layer.  This check in
WebTestProxy was added as a part of converting layout tests to force
compositing mode to make sure that any test asking for a readback was
going down the composited path rather than the legacy software path.

It should be possible to finish a test and ask for an asynchronous
readback prior to having done any compositing updates.  This check does
not seem legit in the first place, so just remove it.

BUG=397321

Review URL: https://codereview.chromium.org/597773002

Cr-Commit-Position: refs/heads/master@{#296450}