Record DRP histogram on how long back savings were cleared

components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 912 matching lines @@
           "Net.DailyContentLength_ViaDataReductionProxy_UnknownMime");
     }
 
     // The system may go backwards in time by up to a day for legitimate
     // reasons, such as with changes to the time zone. In such cases, we
     // keep adding to the current day which is why we check for
     // |days_since_last_update != -1|.
     // 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);
       SetInt64(data_reduction_proxy::prefs::
                    kDailyHttpReceivedContentLengthApplication,
                0);
 
       SetInt64(
           data_reduction_proxy::prefs::kDailyHttpOriginalContentLengthVideo, 0);
       SetInt64(
@@ skipping 303 matching lines @@
 std::string DataReductionProxyCompressionStats::NormalizeHostname(
     const std::string& host) {
   size_t pos = host.find("://");
   if (pos != std::string::npos)
     return host.substr(pos + 3);
 
   return host;
 }
 
 }  // namespace data_reduction_proxy