Try to deflake feImage-target-reappend-to-document

The correct expectation is two repaint rects, but sometimes there is
only one.  I suspect this is because no layout occurs with the set
timeout.  By making this go through a display and layout rather
than just a timeout, I hope that this will fix the flakiness.

R=pdr@chromium.org
BUG=374572

Review URL: https://codereview.chromium.org/292813006

git-svn-id: svn://svn.chromium.org/blink/trunk@174524 bbb929c8-8fbe-4397-9dbb-9b2b20218538