[oilpan]: Fix webkit unit tests for oilpan.

We have to force a GC to ensure the registered event handler are removed. This happens during weak processing with oilpan.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, jochen@chromium.org, oilpan-reviews@chromium.org, rbyers@chromium.org, skyostil@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/391373009

git-svn-id: svn://svn.chromium.org/blink/trunk@178378 bbb929c8-8fbe-4397-9dbb-9b2b20218538