Make client-triggered navigations more friendly to RemoteFrames

      * Change most callers of NavigationScheduler::scheduleLocationChange() to go through Frame::navigate(), which will proxy the navigation request if the target frame is remote.
* Move some helpers from LocalDOMWindow to DOMWindow and teach them about RemoteFrames.
* Move LocalDOMWindow::setLocation() to Location (its only user) and bring it closer to being remote-friendly.

BUG=

Review URL: https://codereview.chromium.org/799903003

git-svn-id: svn://svn.chromium.org/blink/trunk@188452 bbb929c8-8fbe-4397-9dbb-9b2b20218538