Make unload-reparent-sibling-frame.html wait for completion
https://bugs.webkit.org/show_bug.cgi?id=107987

Reviewed by Ojan Vafai.

While this test runs fine locally and in the cr-linux EWS it
appears to be flaky on many of the other bots because it doesn't
wait for the test to complete. Instead make the waiting in the test
explicit.

* fast/frames/unload-reparent-sibling-frame.html:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140885 268f45cc-cd09-0410-ab3c-d52691b4dbfc