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..6c7742ecb3a7e8bb4729518e9f2e5dbd22454aee 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 +482,14 @@ 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);
+  void ComputeInputLatencyHistograms(
+      const char* event_type_name,

[Alexei Svitkine (slow), 2014/10/31 19:00:39]

Can you make this param an enum?

Then, you can switch on it in the body of the function to get the right string.
This way, if someone wants to add another type of entry here, they'll need to
update the enum/string switch, which should have a comment to also update
histograms.xml.

Speaking of which, please add the histograms for Touch to the XML file too.

[ccameron, 2014/10/31 19:09:23]

PS4 had this approach, but that was revised to the current approach.

I've added a comment above ComputeInputLatencyHistogram to indicate that new
entries in the XML file are needed as new event types are used.
The Touch entries are already present (just above the Wheel event entries).

+      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