Trash the OwnPtr pointer in the OwnPtr destructor.

This helps defend against some classes of use-after-free conditions.

No perf change on Parser/html5-full-render

BUG=270545

Review URL: https://chromiumcodereview.appspot.com/23526006

git-svn-id: svn://svn.chromium.org/blink/trunk@157029 bbb929c8-8fbe-4397-9dbb-9b2b20218538