exo: Fix multi-display hardware cursor

ash/wm/ash_native_cursor_manager.cc

Index: ash/wm/ash_native_cursor_manager.cc
diff --git a/ash/wm/ash_native_cursor_manager.cc b/ash/wm/ash_native_cursor_manager.cc
index d79bd5d0af84cb54b780dc2404d4d776b3b381f5..d71d695a95fc8eaf859ac4d521224b2c426966ad 100644
--- a/ash/wm/ash_native_cursor_manager.cc
+++ b/ash/wm/ash_native_cursor_manager.cc
@@ -60,6 +60,14 @@ AshNativeCursorManager::AshNativeCursorManager()
 
 AshNativeCursorManager::~AshNativeCursorManager() {}
 
+void AshNativeCursorManager::AddObserver(Observer* observer) {
+  observers_.AddObserver(observer);
+}
+
+void AshNativeCursorManager::RemoveObserver(Observer* observer) {
+  observers_.RemoveObserver(observer);
+}
+
 void AshNativeCursorManager::SetNativeCursorEnabled(bool enabled) {
   native_cursor_enabled_ = enabled;
 
@@ -71,6 +79,10 @@ void AshNativeCursorManager::SetDisplay(
     const display::Display& display,
     ::wm::NativeCursorManagerDelegate* delegate) {
   DCHECK(display.is_valid());
+
+  for (Observer& observer : observers_)
+    observer.OnCursorDisplayChanging(display);
+
   // Use the platform's device scale factor instead of the display's, which
   // might have been adjusted for the UI scale.
   const float original_scale = Shell::GetInstance()