Remove the compositing/iframes/iframe-src-change.html test.

This test checks that the return value of layerTreeAsText() is not
empty, but it is never empty when we are in force-compositing-mode, so
the test is not testing anything useful anymore.

R=enne@chromium.org
BUG=361729

Review URL: https://codereview.chromium.org/231643003

git-svn-id: svn://svn.chromium.org/blink/trunk@171175 bbb929c8-8fbe-4397-9dbb-9b2b20218538