Fix flake in FCM from tables/mozilla_expected_failures/marvin/table_overflow_dirty_reflow_row.html

The test did not waitUntilDone() and then modified the
dom in js in the onload handler (with a timeout even).
Now the test gets its text and pixel output after
changing the dom.

BUG=370525

Review URL: https://codereview.chromium.org/267123005

git-svn-id: svn://svn.chromium.org/blink/trunk@173423 bbb929c8-8fbe-4397-9dbb-9b2b20218538