Fix canvas blink_perf tests to avoid GPU fallback

There is an artificial regression in some of the canvas performance tests
after landing commit 627ddfd0de197d67567a15191f654b8835408bce
this change fixes the tests to prevent them from triggering the fallback
to CPU-based rendering.

BUG=606688

Review-Url: https://codereview.chromium.org/2126013002
Cr-Commit-Position: refs/heads/master@{#403957}
5 files changed