Deflake fast/images/image-zoom-to-{25,500}.html

The tests were flaky on Linux because resizing the window is async
in GTK+.  If we haven't finished resizing, queue another resize event
so we check again.

BUG=276759
TBR=dominicc@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23242015

git-svn-id: svn://svn.chromium.org/blink/trunk@156531 bbb929c8-8fbe-4397-9dbb-9b2b20218538