This was showing up in profiles of the MotionMark benchmark, possibly
due to re-creating the canvas many times.

BUG=angleproject:1155

Change-Id: Id857b89770b846881fb381f7c3ab70c07b924271
Reviewed-on: https://chromium-review.googlesource.com/655478Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>