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 67ff7ca2eee5792ba26c4f2bac9b60a00a32501a..0e430036f53cdf93f14c5dde63f603a9bb05791c 100644
--- a/content/browser/site_per_process_browsertest.cc
+++ b/content/browser/site_per_process_browsertest.cc
@@ -204,6 +204,9 @@ 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_;