Record DRP histogram on how long back savings were cleared

components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc
index 31a62cc287de196682e27d5291cf28bbb43e2feb..533156d478b4ca3a6882eeca65d44f56f28e715d 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_compression_stats_unittest.cc
@@ -999,6 +999,7 @@ TEST_F(DataReductionProxyCompressionStatsTest, BackwardAndForwardOneDay) {
 }
 
 TEST_F(DataReductionProxyCompressionStatsTest, BackwardTwoDays) {
+  base::HistogramTester histogram_tester;
   const int64_t kOriginalLength = 200;
   const int64_t kReceivedLength = 100;
   int64_t original[] = {kOriginalLength};
@@ -1016,6 +1017,10 @@ TEST_F(DataReductionProxyCompressionStatsTest, BackwardTwoDays) {
       original, 1, received, 1,
       original, 1, received, 1,
       original, 1, received, 1);
+  histogram_tester.ExpectUniqueSample(
+      "DataReductionProxy.CountSavingsCleared.NegativeSystemClock", 1, 1);
+  VerifyPrefInt64(prefs::kDataReductionProxySavingsClearedNegativeSystemClock,
+                  FakeNow().ToInternalValue());
 }
 
 TEST_F(DataReductionProxyCompressionStatsTest, NormalizeHostname) {