Tab dragging as implemented as a mus API.

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 0018ac2b6b0d9e0cb81c85d5b9c325149fd5429b..64e421169c6de0173a9972a0a990ae575e5b6df7 100644
--- a/services/ui/ws/window_server.h
+++ b/services/ui/ws/window_server.h
@@ -196,7 +196,19 @@ class WindowServer : public ServerWindowDelegate,
   // a [re]paint. This should only be called in a test configuration.
   void SetPaintCallback(const base::Callback<void(ServerWindow*)>& callback);
 
+  void StartMoveLoop(uint32_t change_id,
+                     ServerWindow* window,
+                     WindowTree* initiator,
+                     const gfx::Rect& revert_bounds);
+  void EndMoveLoop();
+  uint32_t GetCurrentMoveLoopChangeId();
+  ServerWindow* GetCurrentMoveLoopWindow();
+  WindowTree* GetCurrentMoveLoopInitiator();
+  gfx::Rect GetCurrentMoveLoopRevertBounds();
+  bool in_move_loop() const { return !!current_move_loop_; }
+
  private:
+  struct CurrentMoveLoopState;
   friend class Operation;
 
   using WindowTreeMap =
@@ -311,6 +323,8 @@ class WindowServer : public ServerWindowDelegate,
 
   std::unique_ptr<DisplayManager> display_manager_;
 
+  std::unique_ptr<CurrentMoveLoopState> current_move_loop_;
+
   // Set of WindowTrees.
   WindowTreeMap tree_map_;