mus: Implement interwindow drag and drop

services/ui/public/cpp/tests/test_window_tree.h

Index: services/ui/public/cpp/tests/test_window_tree.h
diff --git a/services/ui/public/cpp/tests/test_window_tree.h b/services/ui/public/cpp/tests/test_window_tree.h
index 94972ea959fdbcddcafe3ea76c4231e7711be84c..1698aceb74a7abcd023cc5e492acfcbea095a297 100644
--- a/services/ui/public/cpp/tests/test_window_tree.h
+++ b/services/ui/public/cpp/tests/test_window_tree.h
@@ -48,6 +48,7 @@ class TestWindowTree : public mojom::WindowTree {
                      mojo::Array<gfx::Rect> additional_client_areas) override;
   void SetHitTestMask(uint32_t window_id,
                       const base::Optional<gfx::Rect>& mask) override;
+  void SetCanAcceptDrags(uint32_t window_id, bool accepts_drags) override;
   void SetWindowVisibility(uint32_t change_id,
                            uint32_t window_id,
                            bool visible) override;
@@ -102,6 +103,12 @@ class TestWindowTree : public mojom::WindowTree {
       override;
   void GetCursorLocationMemory(const GetCursorLocationMemoryCallback& callback)
       override;
+  void PerformDragDrop(uint32_t change_id,
+                       uint32_t source_window_id,
+                       mojo::Map<mojo::String, mojo::Array<uint8_t>> drag_data,
+                       uint32_t drag_operation,
+                       const gfx::Point& cursor_offset,
+                       const SkBitmap& drag_representation) override;
   void PerformWindowMove(uint32_t change_id,
                          uint32_t window_id,
                          mojom::MoveLoopSource source,