Add a sampling scheme to ScrollUpdate.Touch UKM

content/browser/renderer_host/input/render_widget_host_latency_tracker.cc

Index: content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
diff --git a/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc b/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
index 62e42952dd6e9400f3de541511172139ca8b229d..5e5582e5db0cf0d91f049c603e1c59655b486b80 100644
--- a/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
+++ b/content/browser/renderer_host/input/render_widget_host_latency_tracker.cc
@@ -9,6 +9,7 @@
 #include "base/logging.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/rand_util.h"
 #include "build/build_config.h"
 #include "components/rappor/public/rappor_utils.h"
 #include "content/browser/renderer_host/render_widget_host_delegate.h"
@@ -138,7 +139,8 @@ void RecordEQTAccuracy(base::TimeDelta queueing_time,
 
 }  // namespace
 
-RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker()
+RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker(
+    bool metric_sampling)
     : ukm_source_id_(-1),
       last_event_id_(0),
       latency_component_id_(0),
@@ -146,6 +148,7 @@ RenderWidgetHostLatencyTracker::RenderWidgetHostLatencyTracker()
       has_seen_first_gesture_scroll_update_(false),
       active_multi_finger_gesture_(false),
       touch_start_default_prevented_(false),
+      metric_sampling_(metric_sampling),
       render_widget_host_delegate_(nullptr) {}
 
 RenderWidgetHostLatencyTracker::~RenderWidgetHostLatencyTracker() {}
@@ -414,6 +417,12 @@ void RenderWidgetHostLatencyTracker::ReportUkmScrollLatency(
     const LatencyInfo::LatencyComponent& end_component) {
   CONFIRM_VALID_TIMING(start_component, end_component)
 
+  // Only report a subset of this metric as the volume is too high.
+  if (event_name == "Event.ScrollUpdate.Touch") {
+    if (metric_sampling_ && base::RandUint64() % 10)

[tdresser, 2017/07/17 16:26:40]

RandUint64 is thread safe and cryptographically secure. I think we could use
something a bit cheaper here. Maybe even just record every nth value?

[Navid Zolghadr, 2017/07/17 20:15:14]

I changed the implementation to only use random once at the beginning to also
avoid any bias towards any particular cycle.

+      return;
+  }
+
   ukm::SourceId ukm_source_id = GetUkmSourceId();
   ukm::UkmRecorder* ukm_recorder = ukm::UkmRecorder::Get();