Revert "Refactoring: Clarifying DOMWindow ownership by removing Frame::setDOMWindow()"

This reverts r154885 since it introduces a difference in how DOMWindow
is handled if FrameLoader::clear() is re-entered. Before r154885, we
would always assign the frame a new DOMWindow if it wasn't the initial
navigation away from about:blank. After r154885 though, re-entering
FrameLoader::clear() from DocumentLoader::createWriterFor() would end up
reusing the same DOMWindow, since m_needsClear would be false in that
case.

Review URL: https://chromiumcodereview.appspot.com/21296004

git-svn-id: svn://svn.chromium.org/blink/trunk@155226 bbb929c8-8fbe-4397-9dbb-9b2b20218538