Add histograms for observing UMA throttling effect.

components/metrics/persisted_logs.cc

Index: components/metrics/persisted_logs.cc
diff --git a/components/metrics/persisted_logs.cc b/components/metrics/persisted_logs.cc
index c78ecec2596b995dd04364fb9a272ecd5613123c..4282d2ad6432669954dd6f99e5ccb7ec912bbe94 100644
--- a/components/metrics/persisted_logs.cc
+++ b/components/metrics/persisted_logs.cc
@@ -131,11 +131,14 @@ void PersistedLogs::WriteLogsToPrefList(base::ListValue* list_value) const {
     ++saved_log_count;
   }
 
+  UMA_HISTOGRAM_COUNTS("UMA.UnSentLog.Dropped", start - 1);
+
   for (size_t i = start; i < list_.size(); ++i) {
     size_t log_size = list_[i].compressed_log_data.length();
     if (log_size > max_log_size_) {
       UMA_HISTOGRAM_COUNTS("UMA.Large Accumulated Log Not Persisted",
                            static_cast<int>(log_size));
+      UMA_HISTOGRAM_COUNTS("UMA.UnSentLog.Dropped", 1);

[Alexei Svitkine (slow), 2016/04/08 15:08:32]

Can you just log the total at the end of the function? Otherwise, it's strange
to log a value of 1 here and some other value separately.

[gayane -on leave until 09-2017, 2016/04/08 17:19:25]

Done.

       continue;
     }
     AppendBase64String(list_[i].compressed_log_data, list_value);