mus: Implement interwindow drag and drop

services/ui/ws/drag_source.h

Index: services/ui/ws/drag_source.h
diff --git a/services/ui/ws/drag_source.h b/services/ui/ws/drag_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..38f1acf77a0af5def80b2d703be95aea78927a3e
--- /dev/null
+++ b/services/ui/ws/drag_source.h
@@ -0,0 +1,36 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SERVICES_UI_WS_DRAG_SOURCE_H_
+#define SERVICES_UI_WS_DRAG_SOURCE_H_
+
+namespace ui {
+namespace ws {
+
+class DragTargetConnection;
+class ServerWindow;
+struct WindowId;
+
+// An interface implemented by the object that caused a DragController to
+// exist, and which acts as a delegate to it.
+class DragSource {
+ public:
+  virtual ~DragSource() {}
+
+  // Called when a drag operation is completed. |success| is true when a target
+  // window signaled the successful completion of the drag, false in all other
+  // cases where a drag was aborted at any step in the process.
+  virtual void OnDragCompleted(bool success) = 0;
+
+  virtual ServerWindow* GetWindowById(const WindowId& id) = 0;
+
+  // Returns the client connection that created |window|.
+  virtual DragTargetConnection* GetDragTargetForWindow(
+      const ServerWindow* window) = 0;
+};
+
+}  // namespace ws
+}  // namespace ui
+
+#endif  // SERVICES_UI_WS_DRAG_SOURCE_H_