mus: Let clients set the cursor of their window.

components/mus/ws/event_dispatcher.cc

Index: components/mus/ws/event_dispatcher.cc
diff --git a/components/mus/ws/event_dispatcher.cc b/components/mus/ws/event_dispatcher.cc
index c9f71deda563e6948ce4eb61b9a28e9ec3d38429..63b42dd6a9745a2daf6e068a78fd927846c65c9d 100644
--- a/components/mus/ws/event_dispatcher.cc
+++ b/components/mus/ws/event_dispatcher.cc
@@ -141,7 +141,9 @@ class EventMatcher {
 ////////////////////////////////////////////////////////////////////////////////
 
 EventDispatcher::EventDispatcher(EventDispatcherDelegate* delegate)
-    : delegate_(delegate), root_(nullptr) {}
+    : delegate_(delegate),
+      root_(nullptr),
+      last_pointer_target_window_(nullptr) {}
 
 EventDispatcher::~EventDispatcher() {
   std::set<ServerWindow*> pointer_targets;
@@ -263,6 +265,9 @@ void EventDispatcher::DispatchToPointerTarget(const PointerTarget& target,
   if (!target.window)
     return;
 
+  if (event->pointer_data->kind == mojom::PointerKind::POINTER_KIND_MOUSE)
+    last_pointer_target_window_ = target.window;
+
   gfx::Point location(EventLocationToPoint(*event));
   gfx::Transform transform(GetTransformToWindow(surface_id_, target.window));
   transform.TransformPoint(&location);
@@ -313,6 +318,9 @@ void EventDispatcher::OnWindowVisibilityChanged(ServerWindow* window) {
 
 void EventDispatcher::OnWindowDestroyed(ServerWindow* window) {
   CancelPointerEventsToTarget(window);
+
+  if (last_pointer_target_window_ == window)
+    last_pointer_target_window_ = nullptr;
 }
 
 }  // namespace ws