Add a new histogram for EngagementScore to check whether it's zero

chrome/browser/engagement/site_engagement_metrics.cc

Index: chrome/browser/engagement/site_engagement_metrics.cc
diff --git a/chrome/browser/engagement/site_engagement_metrics.cc b/chrome/browser/engagement/site_engagement_metrics.cc
index 83db74ed29d65efa49df7313928f60c65962167c..68723e3e4c7c8ab93a749b0d5b4f51e60bc28e04 100644
--- a/chrome/browser/engagement/site_engagement_metrics.cc
+++ b/chrome/browser/engagement/site_engagement_metrics.cc
@@ -6,6 +6,7 @@
 
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/string_number_conversions.h"
+#include "chrome/browser/engagement/site_engagement_score.h"
 
 namespace {
 
@@ -42,6 +43,9 @@ const char SiteEngagementMetrics::kEngagementScoreHistogramHTTP[] =
 const char SiteEngagementMetrics::kEngagementScoreHistogramHTTPS[] =
     "SiteEngagementService.EngagementScore.HTTPS";
 
+const char SiteEngagementMetrics::kEngagementScoreHistogramZero[] =
+    "SiteEngagementService.EngagementScore.Zero";
+
 const char SiteEngagementMetrics::kOriginsWithMaxEngagementHistogram[] =
     "SiteEngagementService.OriginsWithMaxEngagement";
 
@@ -95,9 +99,11 @@ void SiteEngagementMetrics::RecordEngagementScores(
   for (size_t i = 0; i < arraysize(kEngagementBucketHistogramBuckets); ++i)
     score_buckets[kEngagementBucketHistogramBuckets[i]] = 0;
 
+  const double threshold_0 = SiteEngagementScore::GetScoreCleanupThreshold();

[dominickn, 2017/01/20 02:11:53]

I think you can just compare score == 0 here rather than the cleanup threshold.
Scores can be below the cleanup threshold for an arbitrarily long length of time
(we only cleanup after startup, and if Chrome has been open but idle for some
number of hours, i.e. no engagement is being earned).

[charleszhao, 2017/01/20 03:17:01]

Done.

   for (const auto& value : score_map) {
     double score = value.second;
     UMA_HISTOGRAM_COUNTS_100(kEngagementScoreHistogram, score);
+    UMA_HISTOGRAM_BOOLEAN(kEngagementScoreHistogramZero, score < threshold_0);
     if (value.first.SchemeIs(url::kHttpsScheme)) {
       UMA_HISTOGRAM_COUNTS_100(kEngagementScoreHistogramHTTPS, score);
       https_engagement += score;