Fix "modal isn't modal in  multi displays" issue

This is pretty bad bug because you can do almost everything while system modal dialog is open.

* Generalized the logic to check if the window is above blocking container.

I believe I kept the original behavior otherwise. Please let me know if you noticed something different.

BUG=620406
TEST=manual, covered by tests

Review-Url: https://codereview.chromium.org/2070163002
Cr-Commit-Position: refs/heads/master@{#400589}
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index 535f7d1..1b19fc6 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -168,6 +168,9 @@
   void ShowContextMenu(const gfx::Point& location_in_screen,
                        ui::MenuSourceType source_type);
 
+  // True if the window can receive events on this root window.
+  bool CanWindowReceiveEvents(aura::Window* window);
+
   // Returns the layout-manager for the appropriate modal-container. If the
   // window is inside the lockscreen modal container, then the layout manager
   // for that is returned. Otherwise the layout manager for the default modal