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 96c2f73fe75fe7e9b36912a1be8b19bbab3d5b37..d0ce28e57d8370a97948965e6bedc0215a41403d 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -233,6 +233,17 @@ 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, MoveLoopSource source,
+                    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);

[sky, 2016/06/29 00:04:08]

Can this take change_id, which is meant to uniquely identify the move rather
than window_id?

 };
 
 // Changes to windows are not sent to the connection that originated the