Fix memory use after free error in HTMLCanvasElement tear down

BUG=341093
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/147963006

git-svn-id: svn://svn.chromium.org/blink/trunk@166540 bbb929c8-8fbe-4397-9dbb-9b2b20218538