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Currently in Blink, there is no indication whether a compositor is GPU
accelerated or software. This CL plumbs it to Blink, so that we can
query whether --disable-gpu-compositing or --disable-gpu is set or not.

BUG=650795

Review-Url: https://codereview.chromium.org/2377883002
Cr-Commit-Position: refs/heads/master@{#422672}