DOMWindow::navigator should return a navigator w/o frame when detached.

      Currently DOMWindow::navigator returns a navigator associated with a frame
even when the window is detached from the frame and another window
is attached to it. That means calling frame()->document() may return
an incorrect document, for example.

This CL makes LocalDOMWindow::navigator return a navigator with
a null frame when the window is not associated to the frame.

BUG=522791

Review URL: https://codereview.chromium.org/1308723003

git-svn-id: svn://svn.chromium.org/blink/trunk@201055 bbb929c8-8fbe-4397-9dbb-9b2b20218538