Record DRP histogram on how long back savings were cleared

components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
index 40fbb7882ca0712e4eadf4f47640abc89fde054d..85ad702b32bdbc6787c2469fd2d7bfa2fb7e681a 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc
@@ -930,6 +930,13 @@ void DataReductionProxyCompressionStats::RecordRequestSizePrefs(
     // Note: we accept the fact that some reported data is shifted to
     // the adjacent day if users travel back and forth across time zones.
     if (days_since_last_update && (days_since_last_update != -1)) {
+      if (days_since_last_update < -1) {
+        pref_service_->SetInt64(
+            prefs::kDataReductionProxySavingsClearedNegativeSystemClock,
+            now.ToInternalValue());
+        UMA_HISTOGRAM_EXACT_LINEAR(
+            "DataReductionProxy.CountSavingsCleared.NegativeSystemClock", 1, 2);

[Alexei Svitkine (slow), 2016/12/08 21:24:24]

I think logging this in isolation is not very helpful.

Suppose you have a value like "19383". Is this a lot? Not so much? It doesn't
tell you without knowing something else - like "out of how many users?". It
would be better to record a boolean histogram here for both false and true cases
so you can tell the ratio in the population.

[tbansal1, 2016/12/08 21:47:16]

There are other UMA histograms that record that. The denominator here simply
would be # of Chrome sessions, or # of users (depending on the use case) that
have DRP enabled. I can record this as a boolean but then we are just sending
duplicate data.

+      }
       SetInt64(data_reduction_proxy::prefs::
                    kDailyHttpOriginalContentLengthApplication,
                0);