Factor out latency tracking from RenderWidgetHostImpl

content/browser/renderer_host/event_with_latency_info.h

Index: content/browser/renderer_host/event_with_latency_info.h
diff --git a/content/browser/renderer_host/event_with_latency_info.h b/content/browser/renderer_host/event_with_latency_info.h
index 483c4cffc39c7bd8f28671308bb30a2edcbcc0a0..37f67475b38126d506f45ec00856aec1474ef8fa 100644
--- a/content/browser/renderer_host/event_with_latency_info.h
+++ b/content/browser/renderer_host/event_with_latency_info.h
@@ -22,7 +22,9 @@ template <typename T>
 class EventWithLatencyInfo {
  public:
   T event;
-  ui::LatencyInfo latency;
+  mutable ui::LatencyInfo latency;
+
+  explicit EventWithLatencyInfo(const T& e) : event(e) {}
 
   EventWithLatencyInfo(const T& e, const ui::LatencyInfo& l)
       : event(e), latency(l) {}