Reland: mus: Enable system modal windows

components/mus/ws/event_dispatcher.h

Index: components/mus/ws/event_dispatcher.h
diff --git a/components/mus/ws/event_dispatcher.h b/components/mus/ws/event_dispatcher.h
index 81bf4a09a122d4b6b890e73a342de88e3c5e6176..f9bcdc3336aadfbacde8fbc15d6011731609a4d9 100644
--- a/components/mus/ws/event_dispatcher.h
+++ b/components/mus/ws/event_dispatcher.h
@@ -60,6 +60,15 @@ class EventDispatcher : public ServerWindowObserver {
   // (indicated by returning |false|).
   bool SetCaptureWindow(ServerWindow* capture_window, bool in_nonclient_area);
 
+  ServerWindow* system_modal_window() { return system_modal_window_; }
+  const ServerWindow* system_modal_window() const {
+    return system_modal_window_;
+  }
+
+  // |window| cannot be nullptr. The only way to unset system modal window is by
+  // destroying it.
+  bool SetSystemModalWindow(ServerWindow* window);
+
   // Retrieves the ServerWindow of the last mouse move.
   ServerWindow* mouse_cursor_source_window() const {
     return mouse_cursor_source_window_;
@@ -148,9 +157,11 @@ class EventDispatcher : public ServerWindowObserver {
   // way we continue to eat events until the up/cancel is received.
   void CancelPointerEventsToTarget(ServerWindow* window);
 
-  // Returns true if we're currently an observer for |window|. We are an
-  // observer for a window if any pointer events are targeting it.
-  bool IsObservingWindow(ServerWindow* window);
+  // Used to observe a window. Can be called multiple times on a window. To
+  // unobserve a window, UnobserveWindow() should be called the same number of
+  // times.
+  void ObserveWindow(ServerWindow* winodw);
+  void UnobserveWindow(ServerWindow* winodw);
 
   // Returns an Accelerator bound to the specified code/flags, and of the
   // matching |phase|. Otherwise returns null.
@@ -166,9 +177,12 @@ class EventDispatcher : public ServerWindowObserver {
 
   EventDispatcherDelegate* delegate_;
   ServerWindow* root_;
-  ServerWindow* capture_window_;
 
+  ServerWindow* capture_window_;
   bool capture_window_in_nonclient_area_;
+
+  ServerWindow* system_modal_window_;
+
   bool mouse_button_down_;
   ServerWindow* mouse_cursor_source_window_;
 
@@ -188,6 +202,9 @@ class EventDispatcher : public ServerWindowObserver {
   // cancel or up.
   PointerIdToTargetMap pointer_targets_;
 
+  // Keeps track of number of observe requests for observed windows.
+  std::map<const ServerWindow*, uint8_t> observed_windows_;
+
   DISALLOW_COPY_AND_ASSIGN(EventDispatcher);
 };