Fix crash in Canvas 2D related to GPU reset

This change resolves a crash that occurs when attempting
to release resources held by a 2D canvas that has already
been torn down due to a GL context reset.

BUG=364447
TEST=blink_platform_unittests Canvas2DLayerBridgeTest.testNoDrawOnContextLost

Review URL: https://codereview.chromium.org/495073003

git-svn-id: svn://svn.chromium.org/blink/trunk@180866 bbb929c8-8fbe-4397-9dbb-9b2b20218538