Add input latency histograms for wheel events

content/browser/renderer_host/render_widget_host_impl.h

Index: content/browser/renderer_host/render_widget_host_impl.h
diff --git a/content/browser/renderer_host/render_widget_host_impl.h b/content/browser/renderer_host/render_widget_host_impl.h
index 41df865165ec7b2313b939a62ae5f774dd3edc8f..36134e0d81bdb27557b5542d326bb9b47ff4c50b 100644
--- a/content/browser/renderer_host/render_widget_host_impl.h
+++ b/content/browser/renderer_host/render_widget_host_impl.h
@@ -437,13 +437,12 @@ class CONTENT_EXPORT RenderWidgetHostImpl
   // other way around.
   bool should_auto_resize() { return should_auto_resize_; }
 
-  void ComputeTouchLatency(const ui::LatencyInfo& latency_info);
   void FrameSwapped(const ui::LatencyInfo& latency_info);
   void DidReceiveRendererFrame();
 
   // Returns the ID that uniquely describes this component to the latency
   // subsystem.
-  int64 GetLatencyComponentId();
+  int64 GetLatencyComponentId() const;
 
   static void CompositorFrameDrawn(
       const std::vector<ui::LatencyInfo>& latency_info);
@@ -483,11 +482,17 @@ class CONTENT_EXPORT RenderWidgetHostImpl
   // Create a LatencyInfo struct with INPUT_EVENT_LATENCY_RWH_COMPONENT
   // component if it is not already in |original|. And if |original| is
   // not NULL, it is also merged into the resulting LatencyInfo.
-  ui::LatencyInfo CreateRWHLatencyInfoIfNotExist(
+  ui::LatencyInfo CreateInputEventLatencyInfoIfNotExist(
       const ui::LatencyInfo* original,
       blink::WebInputEvent::Type type,
       const ui::LatencyInfo::InputCoordinate* logical_coordinates,
       size_t logical_coordinates_size);
+  // Add UMA histograms for the latency to the renderer and roundtrip latency
+  // for a given event type. Note that it is necessary to update the histograms
+  // XML file any time histograms for a new event type are added.

[Alexei Svitkine (slow), 2014/10/31 20:18:20]

Can you document in the comment the current strings this is called with? That
way, it's clear when a new one is added that it needs to be updated.

[ccameron, 2014/10/31 22:13:05]

I put a DCHECK that this be "Touch" or "Wheel" next to a comment saying "if you
want more, add them to the XML file".

+  void ComputeInputLatencyHistograms(
+      const char* event_type_name,
+      const ui::LatencyInfo& latency_info) const;
 
   // Called when we receive a notification indicating that the renderer
   // process has gone. This will reset our state so that our state will be