Fix fast/repaint/block-layout-inline-children-replaced.html

The test was loading an image but didn't wait for the image
to fully load to get all the invalidations generated. The
extra 2 invalidations come from the image (92 x 100)
and its container (402 x 100).

The fix involves allowing our invalidation harness to
run asynchronously. While touching the code, removed
some unneeded window-prefix.

BUG=313447
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/184213004

git-svn-id: svn://svn.chromium.org/blink/trunk@168491 bbb929c8-8fbe-4397-9dbb-9b2b20218538