IDL: Use installAttribute() in install_do_not_check_security_signature()

We call SetAccessor() directly in the interface.cpp template, but it
would be better to use V8DOMConfiguration::installAttribute() instead.
Removing one FIXME.

BUG=345503

Review URL: https://codereview.chromium.org/356563005

git-svn-id: svn://svn.chromium.org/blink/trunk@176921 bbb929c8-8fbe-4397-9dbb-9b2b20218538