before invoking on it. not doing this causes a UT test
'VisibleBrowserTest.WindowOpenClose' to crash frequently.
Review URL: http://codereview.chromium.org/17366
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@7905 0039d316-1c4b-4281-b951-d872f2087c98