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 7399841ac3e0d26e9077380f1f2eefaa769e26fc..6ebf6763d0883ff88f72ad5734359f63ac002ae5 100644
--- a/content/browser/site_per_process_browsertest.cc
+++ b/content/browser/site_per_process_browsertest.cc
@@ -208,6 +208,8 @@ class TestInputEventObserver : public RenderWidgetHost::InputEventObserver {
     events_received_.push_back(event.type);
   };
 
+  void OnInputEventAck(const blink::WebInputEvent& event) override {};
+
  private:
   RenderWidgetHost* host_;
   std::vector<blink::WebInputEvent::Type> events_received_;