Add a sampling scheme to ScrollUpdate.Touch UKM

content/browser/renderer_host/input/render_widget_host_latency_tracker.h

Index: content/browser/renderer_host/input/render_widget_host_latency_tracker.h
diff --git a/content/browser/renderer_host/input/render_widget_host_latency_tracker.h b/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
index 00af80ab892464e3a913d9b31034fc52dbc02f14..84aa2554089de73cd8e24d1b1288bdd2132815e5 100644
--- a/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
+++ b/content/browser/renderer_host/input/render_widget_host_latency_tracker.h
@@ -26,7 +26,7 @@ class RenderWidgetHostDelegate;
 class CONTENT_EXPORT RenderWidgetHostLatencyTracker
     : NON_EXPORTED_BASE(public ui::LatencyTracker) {
  public:
-  explicit RenderWidgetHostLatencyTracker();
+  explicit RenderWidgetHostLatencyTracker(bool metric_sampling);
   ~RenderWidgetHostLatencyTracker();
 
   // Associates the latency tracker with a given route and process.
@@ -99,6 +99,12 @@ class CONTENT_EXPORT RenderWidgetHostLatencyTracker
   // default action prevented. Only valid for single finger gestures.
   bool touch_start_default_prevented_;
 
+  // Whether the sampling is needed for high volume metrics. This will be off
+  // when we are in unit tests. This is a temporary field so we can come up with
+  // a more permanent solution for crbug.com/739169.
+  bool metric_sampling_;
+  int metric_sampling_events_since_last_sample_;
+
   RenderWidgetHostDelegate* render_widget_host_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderWidgetHostLatencyTracker);