mus: Introduce API for embedder to dispatch event to the embeded client.

content/renderer/mus/compositor_mus_connection.cc

 // Copyright 2015 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.
 
 #include "content/renderer/mus/compositor_mus_connection.h"
 
 #include "base/single_thread_task_runner.h"
 #include "content/common/input/web_input_event_traits.h"
 #include "content/renderer/input/input_handler_manager.h"
 #include "content/renderer/mus/render_widget_mus_connection.h"
@@ skipping 75 matching lines @@
     ack.Run(EventResult::UNHANDLED);
     return;
   }
   connection->OnWindowInputEvent(std::move(web_event), ack);
 }
 
 void CompositorMusConnection::OnWindowInputEventAckOnMainThread(
     const base::Callback<void(EventResult)>& ack,
     EventResult result) {
   DCHECK(main_task_runner_->BelongsToCurrentThread());
-  compositor_task_runner_->PostTask(FROM_HERE, base::Bind(ack, result));
+  if (!ack.is_null())
+    compositor_task_runner_->PostTask(FROM_HERE, base::Bind(ack, result));
 }
 
 void CompositorMusConnection::OnWindowTreeClientDestroyed(
     mus::WindowTreeClient* client) {
   DCHECK(compositor_task_runner_->BelongsToCurrentThread());
   main_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&CompositorMusConnection::OnConnectionLostOnMainThread, this));
 }
 
@@ skipping 31 matching lines @@
   base::Callback<void(EventResult)> ack =
       base::Bind(&::DoNothingWithEventResult);
   const bool send_ack = WebInputEventTraits::ShouldBlockEventStream(*web_event);
   if (send_ack) {
     // Ultimately, this ACK needs to go back to the Mus client lib which is not
     // thread-safe and lives on the compositor thread. For ACKs that are passed
     // to the main thread we pass them back to the compositor thread via
     // OnWindowInputEventAckOnMainThread.
     ack =
         base::Bind(&CompositorMusConnection::OnWindowInputEventAckOnMainThread,
-                   this, *ack_callback->get());
-    ack_callback->reset();
+                   this,
+                   ack_callback ? *ack_callback->get()
+                                : base::Callback<void(EventResult)>());
+    if (ack_callback)
+      ack_callback->reset();
   }
   main_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&CompositorMusConnection::OnWindowInputEventOnMainThread, this,
                  base::Passed(std::move(web_event)), ack));
 }
 
 }  // namespace content