Add another call to gc() in leak-window-scriptable-object.html to make it less flaky

This is just papering over some GC bug, but it makes the test less flaky. Any test
that uses numberOfLiveNodes is going to be flaky without two gcs, it seems.

R=haraken
BUG=307318,307614

Review URL: https://codereview.chromium.org/27619003

git-svn-id: svn://svn.chromium.org/blink/trunk@159887 bbb929c8-8fbe-4397-9dbb-9b2b20218538