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

components/mus/ws/focus_controller.h

Index: components/mus/ws/focus_controller.h
diff --git a/components/mus/ws/focus_controller.h b/components/mus/ws/focus_controller.h
index 8567f63c1055a3cf13e064f7ac8a9390e557f56e..0b1894efcf28da966488f544c227b44e3ae4be05 100644
--- a/components/mus/ws/focus_controller.h
+++ b/components/mus/ws/focus_controller.h
@@ -13,6 +13,7 @@ namespace mus {
 
 namespace ws {
 
+class FocusControllerDelegate;
 class FocusControllerObserver;
 class ServerWindow;
 class ServerWindowDrawnTracker;
@@ -27,7 +28,7 @@ enum class FocusControllerChangeSource {
 // state of the focused window changes.
 class FocusController : public ServerWindowDrawnTrackerObserver {
  public:
-  FocusController();
+  explicit FocusController(FocusControllerDelegate* delegate);
   ~FocusController() override;
 
   // Sets the focused window. Does nothing if |window| is currently focused.
@@ -35,9 +36,6 @@ class FocusController : public ServerWindowDrawnTrackerObserver {
   void SetFocusedWindow(ServerWindow* window);
   ServerWindow* GetFocusedWindow();
 
-  // Moves activation to the next activatable window.
-  void CycleActivationForward();
-
   void AddObserver(FocusControllerObserver* observer);
   void RemoveObserver(FocusControllerObserver* observer);
 
@@ -46,6 +44,10 @@ class FocusController : public ServerWindowDrawnTrackerObserver {
   bool CanBeFocused(ServerWindow* window) const;
   bool CanBeActivated(ServerWindow* window) const;
 
+  // Returns the closest activatable ancestor of |window|. Returns nullptr if
+  // there is no such ancestor.
+  ServerWindow* GetActivatableAncestorOf(ServerWindow* window) const;
+
   // Implementation of SetFocusedWindow().
   void SetFocusedWindowImpl(FocusControllerChangeSource change_source,
                             ServerWindow* window);
@@ -55,7 +57,14 @@ class FocusController : public ServerWindowDrawnTrackerObserver {
                            ServerWindow* window,
                            bool is_drawn) override;
 
+  FocusControllerDelegate* delegate_;
+
+  ServerWindow* focused_window_;
+  ServerWindow* active_window_;
+
   base::ObserverList<FocusControllerObserver> observers_;
+
+  // Keeps track of the visibility of the focused and active window.
   scoped_ptr<ServerWindowDrawnTracker> drawn_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(FocusController);