Use Frame::domWindow() instead of Frame::document()->domWindow().

Now that Frame holds a direct reference to DOMWindow, just use
domWindow() directly rather than going from the Document back to the
DOMWindow.

BUG=238443
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/18408007

git-svn-id: svn://svn.chromium.org/blink/trunk@154421 bbb929c8-8fbe-4397-9dbb-9b2b20218538