[UserClassifier] Inject a clock

components/ntp_snippets/user_classifier.h

Index: components/ntp_snippets/user_classifier.h
diff --git a/components/ntp_snippets/user_classifier.h b/components/ntp_snippets/user_classifier.h
index 477e23ac760d1c3b9704e2292f8e0bd0b9e06901..93be8fa42e02af8807fd89b0621d0b5980c1f578 100644
--- a/components/ntp_snippets/user_classifier.h
+++ b/components/ntp_snippets/user_classifier.h
@@ -5,14 +5,18 @@
 #ifndef COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_
 #define COMPONENTS_NTP_SNIPPETS_USER_CLASSIFIER_H_
 
+#include <memory>
 #include <string>
 
 #include "base/macros.h"
-#include "base/time/time.h"
 
 class PrefRegistrySimple;
 class PrefService;
 
+namespace base {
+class Clock;
+}  // namespace base
+
 namespace ntp_snippets {
 
 // Collects data about user usage patterns of content suggestions, computes
@@ -52,7 +56,7 @@ class UserClassifier {
   };
 
   // The provided |pref_service| may be nullptr in unit-tests.
-  explicit UserClassifier(PrefService* pref_service);
+  UserClassifier(PrefService* pref_service, std::unique_ptr<base::Clock> clock);
   ~UserClassifier();
 
   // Registers profile prefs for all metrics. Called from browser_prefs.cc.
@@ -93,6 +97,7 @@ class UserClassifier {
   void ClearMetricValue(Metric metric);
 
   PrefService* pref_service_;
+  std::unique_ptr<base::Clock> clock_;
 
   // Params of the metric.
   const double discount_rate_per_hour_;