mus drag and drop: Add API to let the client cancel the drag.

services/ui/ws/window_tree.cc

Index: services/ui/ws/window_tree.cc
diff --git a/services/ui/ws/window_tree.cc b/services/ui/ws/window_tree.cc
index 6ae34e7503a1dd9ec4ba8a99df503a98e5612ccb..1ff99bb39230c92ed806176c73bd5f87c98602bb 100644
--- a/services/ui/ws/window_tree.cc
+++ b/services/ui/ws/window_tree.cc
@@ -1546,6 +1546,26 @@ void WindowTree::PerformDragDrop(
                                std::move(drag_data), drag_operation);
 }
 
+void WindowTree::CancelDragDrop(Id window_id) {
+  ServerWindow* window = GetWindowByClientId(ClientWindowId(window_id));
+  bool success = window && access_policy_->CanInitiateDragLoop(window);

[sky, 2016/09/21 20:40:56]

I wouldn't bother with the success member. That is mostly used as a way to
inform the client of the result, but there is no callback for cancel, so no
point in the member.

Also, DVLOG the error cases please.

+  if (!success)
+    return;
+
+  if (window != window_server_->GetCurrentDragLoopWindow())
+    return;
+
+  if (window_server_->GetCurrentDragLoopInitiator() != this)
+    return;
+
+  WindowManagerDisplayRoot* display_root = GetWindowManagerDisplayRoot(window);
+  if (!display_root)
+    return;
+
+  WindowManagerState* wms = display_root->window_manager_state();
+  wms->CancelDragDrop();
+}
+
 void WindowTree::PerformWindowMove(uint32_t change_id,
                                    Id window_id,
                                    ui::mojom::MoveLoopSource source,