mus: Implement interwindow drag and drop

services/ui/ws/window_server.h

Index: services/ui/ws/window_server.h
diff --git a/services/ui/ws/window_server.h b/services/ui/ws/window_server.h
index 148cda0b2325f4615f79d8a5d382b78794415399..a57816ea9632afb84b4ea88374b611b052d2d720 100644
--- a/services/ui/ws/window_server.h
+++ b/services/ui/ws/window_server.h
@@ -13,6 +13,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "mojo/public/cpp/bindings/array.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "services/ui/clipboard/clipboard_impl.h"
@@ -216,6 +217,11 @@ class WindowServer : public ServerWindowDelegate,
   gfx::Rect GetCurrentMoveLoopRevertBounds();
   bool in_move_loop() const { return !!current_move_loop_; }
 
+  void StartDragLoop(uint32_t change_id);
+  void EndDragLoop();
+  uint32_t GetCurrentDragLoopChangeId();
+  bool in_drag_loop() const { return current_drag_loop_.has_value(); }
+
  private:
   struct CurrentMoveLoopState;
   friend class Operation;
@@ -336,6 +342,8 @@ class WindowServer : public ServerWindowDelegate,
 
   std::unique_ptr<DisplayManager> display_manager_;
 
+  base::Optional<uint32_t> current_drag_loop_;
+
   std::unique_ptr<CurrentMoveLoopState> current_move_loop_;
 
   // Set of WindowTrees.