Fix crash in Canvas2DLayerBridge after a GPU resource is lost

BUG=411864
TEST=blink unit test Canvas2DLayerBridgeTest.testPrepareMailboxAndLoseResource

Review URL: https://codereview.chromium.org/552423002

git-svn-id: svn://svn.chromium.org/blink/trunk@181679 bbb929c8-8fbe-4397-9dbb-9b2b20218538