Delay Input.dispatchKeyEvent response until after key event ack.

content/browser/site_per_process_browsertest.cc

Index: content/browser/site_per_process_browsertest.cc
diff --git a/content/browser/site_per_process_browsertest.cc b/content/browser/site_per_process_browsertest.cc
index df3ee7c607886cd4dbb2fc7f39aea1e8b8103648..069921cf125949b6c2afa5119d0017f0d5ae7cba 100644
--- a/content/browser/site_per_process_browsertest.cc
+++ b/content/browser/site_per_process_browsertest.cc
@@ -203,6 +203,9 @@ class TestInputEventObserver : public RenderWidgetHost::InputEventObserver {
     events_received_.push_back(event.type);
   };
 
+  void OnInputEventAck(const blink::WebInputEvent& event, bool is_synthetic)
+      override {};
+
  private:
   RenderWidgetHost* host_;
   std::vector<blink::WebInputEvent::Type> events_received_;