Record UMA on number of days since data reduction proxy is enabled.

components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h b/components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h
index a6ee82792e689c61c9ffdf14683226f8ee302c18..dcbc20d4ea9b2e7546e2282bda36e40569738ae9 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_settings.h
@@ -9,7 +9,6 @@
 
 #include <memory>
 #include <string>
-#include <vector>
 
 #include "base/callback.h"
 #include "base/compiler_specific.h"
@@ -24,6 +23,10 @@
 
 class PrefService;
 
+namespace base {
+class Clock;
+}
+
 namespace data_reduction_proxy {
 
 class DataReductionProxyConfig;
@@ -257,6 +260,12 @@ class DataReductionProxySettings : public DataReductionProxyServiceObserver,
                            TestLoFiSessionStateHistograms);
   FRIEND_TEST_ALL_PREFIXES(DataReductionProxySettingsTest,
                            TestSettingsEnabledStateHistograms);
+  FRIEND_TEST_ALL_PREFIXES(DataReductionProxySettingsTest,
+                           TestDaysSinceEnabled);
+  FRIEND_TEST_ALL_PREFIXES(DataReductionProxySettingsTest,
+                           TestDaysSinceEnabledWithTestClock);
+  FRIEND_TEST_ALL_PREFIXES(DataReductionProxySettingsTest,
+                           TestDaysSinceEnabledExistingUser);
 
   // Override of DataReductionProxyService::Observer.
   void OnServiceInitialized() override;
@@ -325,6 +334,9 @@ class DataReductionProxySettings : public DataReductionProxyServiceObserver,
 
   SyntheticFieldTrialRegistrationCallback register_synthetic_field_trial_;
 
+  // Should not be null.
+  std::unique_ptr<base::Clock> clock_;
+
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(DataReductionProxySettings);