Fix DOMWindow::isCurrentlyDisplayedInFrame check.

postMessage's timer callback previously used two criteria
to determine if the target DOMWindow was still active:

1) document() is not null. Unfortunately, this no longer
    works as expected since document() is only cleared in
   ~DOMWindow, and the postMessage timer takes a reference
   to DOMWindow.

2) If something is holding a reference to Frame, then the
   call to ~Frame may not occur immediately after the frame
   is detached, and its DOMWindow will still point to this.

To prevent the timer from having an effect on a detached
frame, we must also verify that the frame still has a
FrameHost.

BUG=331904
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/132653002

git-svn-id: svn://svn.chromium.org/blink/trunk@164837 bbb929c8-8fbe-4397-9dbb-9b2b20218538