Add a basic DOMWindow base class and use it in WindowProxy.

This is the first step towards providing RemoteFrames with a
DOMWindow so things like postMessage() on a remote frame will work.

This patch also moves Frame::setDOMWindow with a LocalDOMWindow to
LocalFrame, since it's only called on a LocalFrame. The
corresponding member has also been moved to LocalFrame, since it
was never set to a non-null value for RemoteFrames anyway.

BUG=425623

Review URL: https://codereview.chromium.org/640803004

git-svn-id: svn://svn.chromium.org/blink/trunk@184244 bbb929c8-8fbe-4397-9dbb-9b2b20218538