Oilpan: avoid leak on creating shadow document wrapper.

BUG=357163
TBR=haraken@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/289283020

git-svn-id: svn://svn.chromium.org/blink/trunk@174582 bbb929c8-8fbe-4397-9dbb-9b2b20218538