mus: Implement interwindow drag and drop

services/ui/ws/test_utils.h

Index: services/ui/ws/test_utils.h
diff --git a/services/ui/ws/test_utils.h b/services/ui/ws/test_utils.h
index 742fcf0356ded0009ddd0a1897970414b922437f..eb2c00a36323c7e1d4cf7512f58c328b8ee6937a 100644
--- a/services/ui/ws/test_utils.h
+++ b/services/ui/ws/test_utils.h
@@ -410,6 +410,23 @@ class TestWindowTreeClient : public ui::mojom::WindowTreeClient {
   void OnWindowFocused(uint32_t focused_window_id) override;
   void OnWindowPredefinedCursorChanged(uint32_t window_id,
                                        mojom::Cursor cursor_id) override;
+  void OnDragEnter(uint32_t window,
+                   mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
+                   uint32_t key_state,
+                   const gfx::Point& position,
+                   uint32_t effect_bitmask,
+                   const OnDragEnterCallback& callback) override;
+  void OnDragOver(uint32_t window,
+                  uint32_t key_state,
+                  const gfx::Point& position,
+                  uint32_t effect_bitmask,
+                  const OnDragOverCallback& callback) override;
+  void OnDragLeave(uint32_t window) override;
+  void OnDragDrop(uint32_t window,
+                  uint32_t key_state,
+                  const gfx::Point& position,
+                  uint32_t effect_bitmask,
+                  const OnDragDropCallback& callback) override;
   void OnChangeCompleted(uint32_t change_id, bool success) override;
   void RequestClose(uint32_t window_id) override;
   void GetWindowManager(