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

content/renderer/mus/render_widget_mus_connection.cc

Index: content/renderer/mus/render_widget_mus_connection.cc
diff --git a/content/renderer/mus/render_widget_mus_connection.cc b/content/renderer/mus/render_widget_mus_connection.cc
index e146893e43e16fde309afd1d42f74f78162d6b80..e735a77b1b172b80682c70c57a430e79cb4703d5 100644
--- a/content/renderer/mus/render_widget_mus_connection.cc
+++ b/content/renderer/mus/render_widget_mus_connection.cc
@@ -112,6 +112,8 @@ void RenderWidgetMusConnection::OnDidOverscroll(
 
 void RenderWidgetMusConnection::OnInputEventAck(
     std::unique_ptr<InputEventAck> input_event_ack) {
+  if (pending_ack_.is_null())
+    return;
   DCHECK(!pending_ack_.is_null());
   pending_ack_.Run(input_event_ack->state ==
                            InputEventAckState::INPUT_EVENT_ACK_STATE_CONSUMED
@@ -127,7 +129,7 @@ void RenderWidgetMusConnection::NotifyInputEventHandled(
 
 void RenderWidgetMusConnection::SetInputHandler(
     RenderWidgetInputHandler* input_handler) {
-  DCHECK(!input_handler_);
+  DCHECK(!input_handler_ || !input_handler);
   input_handler_ = input_handler;
 }
 
@@ -160,6 +162,7 @@ void RenderWidgetMusConnection::OnWindowInputEvent(
     std::unique_ptr<blink::WebInputEvent> input_event,
     const base::Callback<void(mus::mojom::EventResult)>& ack) {
   DCHECK(thread_checker_.CalledOnValidThread());
+  LOG(ERROR) << input_event->type;
   // If we don't yet have a RenderWidgetInputHandler then we don't yet have
   // an initialized RenderWidget.
   if (!input_handler_) {