Tab dragging as implemented as a mus API.

components/mus/public/cpp/window_tree_client.h

Index: components/mus/public/cpp/window_tree_client.h
diff --git a/components/mus/public/cpp/window_tree_client.h b/components/mus/public/cpp/window_tree_client.h
index f22e3840632a793f7da52bac3f3456784cf59587..ca0555eb51dc9368331d4bee5c44e1f2861f3e58 100644
--- a/components/mus/public/cpp/window_tree_client.h
+++ b/components/mus/public/cpp/window_tree_client.h
@@ -180,6 +180,14 @@ class WindowTreeClient : public mojom::WindowTreeClient,
   // will be ignored.
   void SetEventObserver(mojom::EventMatcherPtr matcher);
 
+  // Performs a window move. This call blocks but spins up a nested message
+  // loop to handle incoming move messages.
+  bool PerformWindowMove(Window* window, const gfx::Point& cursor_location);
+
+  // Cancels a in progress window move. (If no window is currently being moved,
+  // does nothing.)
+  void CancelWindowMove(Window* window);
+
   // Creates and returns a new Window (which is owned by the window server).
   // Windows are initially hidden, use SetVisible(true) to show.
   Window* NewWindow() { return NewWindow(nullptr); }
@@ -299,6 +307,7 @@ class WindowTreeClient : public mojom::WindowTreeClient,
   void OnWindowPredefinedCursorChanged(Id window_id,
                                        mojom::Cursor cursor) override;
   void OnChangeCompleted(uint32_t change_id, bool success) override;
+  void OnMoveLoopCompleted(uint32_t change_id, bool success) override;
   void RequestClose(uint32_t window_id) override;
   void GetWindowManager(
       mojo::AssociatedInterfaceRequest<WindowManager> internal) override;
@@ -321,6 +330,10 @@ class WindowTreeClient : public mojom::WindowTreeClient,
                                   transport_properties) override;
   void WmClientJankinessChanged(ClientSpecificId client_id,
                                 bool janky) override;
+  void WmPerformMoveLoop(uint32_t change_id,
+                         Id window_id,
+                         const gfx::Point& cursor_location) override;
+  void WmCancelMoveLoop(uint32_t window_id) override;
   void OnAccelerator(uint32_t id, std::unique_ptr<ui::Event> event) override;
 
   // Overridden from WindowManagerClient:
@@ -339,6 +352,9 @@ class WindowTreeClient : public mojom::WindowTreeClient,
       Window* window,
       const gfx::Vector2d& offset,
       const gfx::Insets& hit_area) override;
+  void OnWmMoveLoopCompleted(uint32_t change_id,
+                             uint32_t window_id,
+                             bool completed) override;
 
   // The one int in |cursor_location_mapping_|. When we read from this
   // location, we must always read from it atomically.
@@ -396,6 +412,11 @@ class WindowTreeClient : public mojom::WindowTreeClient,
   // Monotonically increasing ID for event observers.
   uint32_t event_observer_id_ = 0u;
 
+  // We spin up a nested message loop to handle interactive window moves; this
+  // is the return value and the Closure to call to complete the loop.
+  bool current_move_succeeded_ = false;
+  base::Closure on_current_move_finished_;
+
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);