mus: Implement interwindow drag and drop

services/ui/public/interfaces/window_tree_constants.mojom

 // 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.
 
 module ui.mojom;
 
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
 // Contains state of a single window.
 struct WindowData {
@@ skipping 29 matching lines @@
   // in the Window than the owner may also render to this surface as well.
   DEFAULT,
 };
 
 // The result of an input event sent to a client app.
 enum EventResult {
   HANDLED,
   UNHANDLED,
 };
 
+// A bitfield of what drag operations are possible.
+const uint32 kDropEffectNone = 0;
+const uint32 kDropEffectMove = 1;
+const uint32 kDropEffectCopy = 2;
+const uint32 kDropEffectLink = 4;
+
 // When this flag is set in a call to Embed(), the embedder (i.e. the client
 // that is making the call to Embed()) will receive events that are targeted to
 // the embedded client. The embedded client will not receive any input events
 // from the window server. However, the embedder can choose to dispatch events
 // to the embedded client through other mechanism.
 // TODO(sad): Provide an API in mus for the embedder to dispatch events to the
 // embedded client. https://crbug.com/621085
 const uint32 kEmbedFlagEmbedderInterceptsEvents = 0x01;