mus: Allow embedder to intercept events.

components/mus/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 mus.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,
 };
 
+// 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;