Fixing 'http/tests/security/cross-frame-access-port-explicit-domain.html'.

This test periodically flakes on all platforms: the apparent cause is the
iframe's message getting processed by the main frame before the subframe
has actually finished loading.

This patch moves the postMessage call down below the element that the
main frame looks for; that should take care of the flake.

BUG=260236
R=marja@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19178002

git-svn-id: svn://svn.chromium.org/blink/trunk@154221 bbb929c8-8fbe-4397-9dbb-9b2b20218538