Rework import-in-iframe to no longer flake

The test relied on the order imports were resolved, which is not
guaranteed. Now running the test after all imports are resolved.

BUG=295789

Review URL: https://codereview.chromium.org/26554003

git-svn-id: svn://svn.chromium.org/blink/trunk@159171 bbb929c8-8fbe-4397-9dbb-9b2b20218538