Remove iframe@seamless

This CL removes support for seamless iframes. Seamless iframes were already
behind a runtime flag, which means this CL doesn't have any web exposed
effects. Hopefully someday we'll return to iframe@seamless, but we're currently
investing our efforts elsewhere.

Review URL: https://codereview.chromium.org/138443013

git-svn-id: svn://svn.chromium.org/blink/trunk@165781 bbb929c8-8fbe-4397-9dbb-9b2b20218538