Add WARN_UNUSED_RESULT to base::Time methods that return bool.

components/metrics/data_use_tracker_unittest.cc

 // Copyright 2016 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/metrics/data_use_tracker.h"
 
 #include "base/strings/stringprintf.h"
 #include "components/metrics/metrics_pref_names.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/testing_pref_service.h"
@@ skipping 33 matching lines @@
     return true;
   }
 
   bool GetUmaRatio(double* ratio) const override {
     *ratio = 0.05;
     return true;
   }
 
   base::Time GetCurrentMeasurementDate() const override {
     base::Time today_for_test;
-    base::Time::FromUTCString(kTodayStr, &today_for_test);
-    return today_for_test;
+    if (base::Time::FromUTCString(kTodayStr, &today_for_test)) {
+      return today_for_test;
+    } else {
+      return {};
+    }
   }
 
   std::string GetCurrentMeasurementDateAsString() const override {
     return kTodayStr;
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FakeDataUseTracker);
 };
 
@@ skipping 131 matching lines @@
   can_upload = data_use_tracker.ShouldUploadLogOnCellular(100);
   EXPECT_TRUE(can_upload);
   // this is about 0.49%
   can_upload = data_use_tracker.ShouldUploadLogOnCellular(200);
   EXPECT_TRUE(can_upload);
   can_upload = data_use_tracker.ShouldUploadLogOnCellular(300);
   EXPECT_FALSE(can_upload);
 }
 
 }  // namespace metrics