Fix ash::IsToplevelWindow to support window's parent existence

I've just noticed that crrev.com/606339 causes a crash, and that's
because PrintWindowHierarchy starts from a root window, which
do not have a parent, but it tries to call IsToplevelWindow() which
assumes the existence of the parent.

BUG=none
TEST=manually

Change-Id: Ie4460302674d057a6c7960e0b012c80b63e06a2c
Reviewed-on: https://chromium-review.googlesource.com/c/1327560Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606651}

ash/wm/focus_rules.cc

@@ -16,7 +16,7 @@ namespace ash {
 bool IsToplevelWindow(aura::Window* window) {
   DCHECK(window);
   // The window must in a valid hierarchy.
-  if (!window->GetRootWindow())
+  if (!window->GetRootWindow() || !window->parent())
     return false;
 
   // The window must exist within a container that supports activation.