Prevent the load event from firing inside DOMContentLoaded

BUG=406422
TEST=Updated results to fast/loader/create-frame-in-DOMContentLoaded.html

Review URL: https://codereview.chromium.org/725593004

git-svn-id: svn://svn.chromium.org/blink/trunk@185397 bbb929c8-8fbe-4397-9dbb-9b2b20218538