Tab dragging as implemented as a mus API.

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index 878b93f556280c41e2517690f222324ec3e4abd1..d1692baddb89f50a5e550589e5fb7bf627e9c57d 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -229,6 +229,16 @@ interface WindowTree {
   // single Atomic32, which represent the current location of the mouse cursor
   // where the location is (x << 16) | y.
   GetCursorLocationMemory() => (handle<shared_buffer> cursor_buffer);
+
+  // Tells the window manager to start moving the window. OnChangeCompleted is
+  // called on whether the move was canceled. Because there's a delay between
+  // when a client sends this message and when the window manager starts acting
+  // on it, pass the cursor location at the start of the move.
+  PerformWindowMove(uint32 change_id, uint32 window_id, gfx.mojom.Point cursor);
+
+  // Tells the window manager to cancel any in progress window move started with
+  // StartWindowMove() and to revert the window bounds to how they were.
+  CancelWindowMove(uint32 window_id);
 };
 
 // Changes to windows are not sent to the connection that originated the
@@ -358,6 +368,9 @@ interface WindowTreeClient {
   // change ids for details.
   OnChangeCompleted(uint32 change_id, bool success);
 
+  // Called in response to PerformWindowMove().
+  OnMoveLoopCompleted(uint32 change_id, bool success);

[sky, 2016/06/24 19:59:00]

Seem my comment in other patch, I think you should use OnChangeCompleted and the
client can figure out if it corresponds to a move.

+
   // The WindowManager is requesting the specified window to close. If the
   // client allows the change it should delete the window.
   RequestClose(uint32 window_id);