mus: Implement interwindow drag and drop

services/ui/public/cpp/window_drop_target.h

Index: services/ui/public/cpp/window_drop_target.h
diff --git a/services/ui/public/cpp/window_drop_target.h b/services/ui/public/cpp/window_drop_target.h
new file mode 100644
index 0000000000000000000000000000000000000000..0ff0c6cdf87f3e0f93c48b97795039a8068e2634
--- /dev/null
+++ b/services/ui/public/cpp/window_drop_target.h
@@ -0,0 +1,65 @@
+// 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_PUBLIC_CPP_WINDOW_DROP_TARGET_H_
+#define SERVICES_UI_PUBLIC_CPP_WINDOW_DROP_TARGET_H_
+
+#include <map>
+#include <vector>
+
+#include "ui/gfx/geometry/point.h"
+
+namespace ui {
+
+// Interface that clients that want to opt-in to receiving drag drop events
+// pass to their ui::Window. This is the client side equivalent to the
+// messages defined in ui::mojom::WindowTreeClient.
+class WindowDropTarget {
+ public:
+  // On the first time the pointer enters the associated ui::Window, we get a
+  // start message with all the data that's part of the drag. (The source
+  // ui::Window receives this immediately through the client library instead
+  // of asynchronously.)
+  virtual void OnDragDropStart(
+      std::map<std::string, std::vector<uint8_t>> mime_data) = 0;
+
+  // Each time the pointer enters the associated ui::Window, we receive an
+  // enter event and return a bitmask of drop operations that can be performed
+  // at this location, in terms of the ui::mojom::kDropEffect{None,Move,
+  // Copy,Link} constants.
+  virtual uint32_t OnDragEnter(uint32_t event_flags,
+                               const gfx::Point& position,
+                               uint32_t effect_bitmask) = 0;
+
+  // Each time the pointer moves inside the associated ui::Window, we receive
+  // an over event and return a bitmask of drop oeprations.
+  virtual uint32_t OnDragOver(uint32_t event_flags,
+                              const gfx::Point& position,
+                              uint32_t effect_bitmask) = 0;
+
+  // Each time the pointer leaves the associated ui::Window, we receive a
+  // leave event.
+  virtual void OnDragLeave() = 0;
+
+  // If the user releases the pointer over the associated ui::Window, we
+  // receive this drop event to actually try to perform the drop. Unlike the
+  // other methods in this class which return a value, this return value is not
+  // a bitmask and is the operation which was actually completed. Returning 0
+  // indicates that the drag failed and shouldn't be reported as a success.
+  virtual uint32_t OnCompleteDrop(uint32_t event_flags,
+                                  const gfx::Point& position,
+                                  uint32_t effect_bitmask) = 0;
+
+  // When a drag that entered the associated window finishes one way or
+  // another, the target receives this message to clear the mime_data from the
+  // start message, along with any other client specific caches.
+  virtual void OnDragDropDone() = 0;
+
+ protected:
+  virtual ~WindowDropTarget() {}
+};
+
+}  // namespace ui
+
+#endif  // SERVICES_UI_PUBLIC_CPP_WINDOW_DROP_TARGET_H_