mus: Implement interwindow drag and drop

services/ui/ws/window_tree.h

Index: services/ui/ws/window_tree.h
diff --git a/services/ui/ws/window_tree.h b/services/ui/ws/window_tree.h
index 2f0f9445598ac00942deb714d87b5bc77c6697d2..b187f9ec7c648b7e5ffac2b7a406fa141581758c 100644
--- a/services/ui/ws/window_tree.h
+++ b/services/ui/ws/window_tree.h
@@ -193,6 +193,27 @@ class WindowTree : public mojom::WindowTree,
   // window(s).
   void ClientJankinessChanged(WindowTree* tree);
 
+  // The following methods send drag and drop messages to the client and return
+  // their results through the passed in callback.
+  void PerformOnDragEnter(
+      const ServerWindow* window,
+      mojo::Map<mojo::String, mojo::Array<uint8_t>> mime_data,
+      uint32_t key_state,
+      const gfx::Point& cursor_offset,
+      uint32_t effect_bitmask,
+      const base::Callback<void(uint32_t)>& callback);
+  void PerformOnDragOver(const ServerWindow* window,
+                         uint32_t key_state,
+                         const gfx::Point& cursor_offset,
+                         uint32_t effect_bitmask,
+                         const base::Callback<void(uint32_t)>& callback);
+  void PerformOnDragLeave(const ServerWindow* window);
+  void PerformOnDragDrop(const ServerWindow* window,
+                         uint32_t key_state,
+                         const gfx::Point& cursor_offset,
+                         uint32_t effect_bitmask,
+                         const base::Callback<void(uint32_t)>& callback);
+
   // The following methods are invoked after the corresponding change has been
   // processed. They do the appropriate bookkeeping and update the client as
   // necessary.
@@ -418,6 +439,7 @@ class WindowTree : public mojom::WindowTree,
       Id transport_window_id,
       const gfx::Insets& insets,
       mojo::Array<gfx::Rect> transport_additional_client_areas) override;
+  void SetCanAcceptDrags(Id window_id, bool accepts_drags) override;
   void SetHitTestMask(Id transport_window_id,
                       const base::Optional<gfx::Rect>& mask) override;
   void GetWindowManagerClient(
@@ -425,6 +447,12 @@ class WindowTree : 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,
                          Id window_id,
                          ui::mojom::MoveLoopSource source,