Eliminate globalFlag usage from http security layout tests.

Tests that polled globalFlag were using it as a way to figure out when
another subframe had loaded. Instead of using globalFlag, those tests
have been rewritten to use a combination of the window load event (which
waits for all subframes to finish loading before firing), the iframe
load event (which also waits for all subframes), and postMessage when
a subframe can only start running its tests after the entire page is
loaded.

BUG=405846
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/494343003

git-svn-id: svn://svn.chromium.org/blink/trunk@180782 bbb929c8-8fbe-4397-9dbb-9b2b20218538