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

content/renderer/mus/compositor_mus_connection.cc

Index: content/renderer/mus/compositor_mus_connection.cc
diff --git a/content/renderer/mus/compositor_mus_connection.cc b/content/renderer/mus/compositor_mus_connection.cc
index 322fa11becb9ee19602bb095370a1fadcb52fd9f..95b0e546a85c73dddc27d2ac9efb7287eae44589 100644
--- a/content/renderer/mus/compositor_mus_connection.cc
+++ b/content/renderer/mus/compositor_mus_connection.cc
@@ -93,7 +93,8 @@ 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(
@@ -145,8 +146,11 @@ void CompositorMusConnection::OnWindowInputEvent(
     // 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,