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

This chagne replaces Frame::setDOMWindow() by Frame::clearDOMWindow() and
Frame::ensureDOMWindow() to clarify the owner of DOMWindow objects.
Now it is created and owned by Frame.

BUG=none
TEST=none
R=abarth, japhet

Review URL: https://chromiumcodereview.appspot.com/19798011

git-svn-id: svn://svn.chromium.org/blink/trunk@154885 bbb929c8-8fbe-4397-9dbb-9b2b20218538