mus: Allow the WM to specify the windows that can have active children.

components/mus/ws/window_tree_host_impl.h

Index: components/mus/ws/window_tree_host_impl.h
diff --git a/components/mus/ws/window_tree_host_impl.h b/components/mus/ws/window_tree_host_impl.h
index dd7f00323b892af5e366adb9bc9fd53092f7137c..0284948d4c8d54d2651d18805f8a56a414a4b674 100644
--- a/components/mus/ws/window_tree_host_impl.h
+++ b/components/mus/ws/window_tree_host_impl.h
@@ -94,6 +94,7 @@ class WindowTreeHostImpl : public DisplayManagerDelegate,
   void AddAccelerator(uint32_t id,
                       mojom::EventMatcherPtr event_matcher) override;
   void RemoveAccelerator(uint32_t id) override;
+  void SetActivationParent(uint32_t window_id, bool enable) override;
 
  private:
   void OnClientClosed();
@@ -109,6 +110,9 @@ class WindowTreeHostImpl : public DisplayManagerDelegate,
   void OnCompositorFrameDrawn() override;
 
   // FocusControllerDelegate:
+  bool CanHaveActiveChildren(ServerWindow* window) const override;
+  void OnActivationChanged(ServerWindow* old_active_window,
+                           ServerWindow* new_active_window) override;
   void OnFocusChanged(ServerWindow* old_focused_window,
                       ServerWindow* new_focused_window) override;
 
@@ -132,6 +136,8 @@ class WindowTreeHostImpl : public DisplayManagerDelegate,
   scoped_ptr<FocusController> focus_controller_;
   mojom::WindowManagerPtr window_manager_;
 
+  std::set<WindowId> activation_parents_;
+
   // Set of windows with surfaces that need to be destroyed once the frame
   // draws.
   std::set<ServerWindow*> windows_needing_frame_destruction_;