Fix flaky crash in virtual/gpu/fast/canvas/canvas-lost-gpu-context.html

This change fixes a debug assertion that was vulnerable to a race condition.

BUG=327698

Review URL: https://codereview.chromium.org/233263003

git-svn-id: svn://svn.chromium.org/blink/trunk@171290 bbb929c8-8fbe-4397-9dbb-9b2b20218538