Deflake LayoutTests/http/tests/security/cross-origin-appcache-allowed.html

The iframe load was racing with the main frame setting the onmessage
event handler. To fix this, defer the iframe loads until the main frame
onmessage event handler has already been set.

BUG=none

Review URL: https://codereview.chromium.org/704413010

git-svn-id: svn://svn.chromium.org/blink/trunk@185150 bbb929c8-8fbe-4397-9dbb-9b2b20218538