IDL: Add CheckSecurity=Window

To reduce special handlding for EventTarget in the CG. Since
LocalDOMWindow already has frame() method and LocalDOMWindow inherits
from EventTarget, we can't add EventTarget::frame() in an efficient
way. This means that we can't merge CheckSecurity=Frame and
CheckSecurity=Window.

This CL doesn't change generated code.

BUG=383699

Review URL: https://codereview.chromium.org/491053003

git-svn-id: svn://svn.chromium.org/blink/trunk@180766 bbb929c8-8fbe-4397-9dbb-9b2b20218538