HTML Imports: Fix hidden flakiness on tests

Some tests depends on a FOUC behavior and should be fixed to
wait a renderable frame.

BUG=370490
R=esprehn@chromium.org, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/269313002

git-svn-id: svn://svn.chromium.org/blink/trunk@173476 bbb929c8-8fbe-4397-9dbb-9b2b20218538