'security/xss-DENIED-mime-type-execute-as-html.html' times out.

This test loads 15 iframes, which is crazy slow on all platforms (times
out on Windows, loads in ~10 seconds on Mac). This patch splits the test
into 15 single-iframe tests, which should speed things up nicely.

BUG=42044
R=jochen@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19150003

git-svn-id: svn://svn.chromium.org/blink/trunk@154207 bbb929c8-8fbe-4397-9dbb-9b2b20218538