Fixing flaky crash in Canvas2DLayerBridge tear-down.

R=senorblanco
BUG=269625
TEST=manual (see bug description)

Review URL: https://chromiumcodereview.appspot.com/22545006

git-svn-id: svn://svn.chromium.org/blink/trunk@155798 bbb929c8-8fbe-4397-9dbb-9b2b20218538