Add leak-window-scriptable-object test.

Re-land the window scriptable object leak test from crrev.com/14019005.

This test was flaky when originally added, so re-landing it with flaky test
expectations to avoid closing pending it being made more reliable.

BUG=152006

Review URL: https://chromiumcodereview.appspot.com/19081002

git-svn-id: svn://svn.chromium.org/blink/trunk@154085 bbb929c8-8fbe-4397-9dbb-9b2b20218538