Oilpan: avoid leak on creating shadow document wrapper.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@174582 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs