mus: Implement interwindow drag and drop

services/ui/public/cpp/window.h

Index: services/ui/public/cpp/window.h
diff --git a/services/ui/public/cpp/window.h b/services/ui/public/cpp/window.h
index fc5efd9eb419511e94ebba6c6767a080399af3c9..ada2cf63b6d41fa6b66259dde8fe81c4522f84da 100644
--- a/services/ui/public/cpp/window.h
+++ b/services/ui/public/cpp/window.h
@@ -30,6 +30,7 @@ class ServiceProviderImpl;
 class WindowObserver;
 class WindowSurface;
 class WindowSurfaceBinding;
+class WindowDropTarget;
 class WindowTreeClient;
 class WindowTreeClientPrivate;
 
@@ -209,6 +210,11 @@ class Window {
   bool HasFocus() const;
   void SetCanFocus(bool can_focus);
 
+  // Sets whether this window accepts drags. Passing a non-null |drop_target|
+  // will enable acceptance of drops. Passing null will disable it.
+  void SetCanAcceptDrops(WindowDropTarget* drop_target);
+  WindowDropTarget* drop_target() { return drop_target_; }
+
   // Sets whether this window accepts events.
   void SetCanAcceptEvents(bool can_accept_events);
 
@@ -225,6 +231,15 @@ class Window {
   // to a better place.
   void RequestClose();
 
+  // Starts an inter-process drag and drop operation.
+  void PerformDragDrop(
+      int drag_pointer,
+      const std::map<std::string, std::vector<uint8_t>>& drag_data,
+      int drag_operation,
+      const gfx::Point& cursor_location,
+      const SkBitmap& bitmap,
+      const base::Callback<void(bool)>& callback);
+
   // Tells the window manager to take control of moving the window. Returns
   // true if the move wasn't canceled.
   void PerformWindowMove(mojom::MoveLoopSource source,
@@ -342,6 +357,10 @@ class Window {
   float opacity_;
   int64_t display_id_;
 
+  // The client supplied delegate that receives drag events for this
+  // window (weak ptr).
+  WindowDropTarget* drop_target_ = nullptr;
+
   // Whether this window can accept events. Initialized to true to
   // match ServerWindow.
   bool can_accept_events_ = true;