Change uses of base::JoinString to pass StringPieces where possible.

components/safe_browsing_db/safe_browsing_prefs_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 <string>
 #include <vector>
 
 #include "base/command_line.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/test/scoped_feature_list.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/testing_pref_service.h"
 #include "components/safe_browsing_db/safe_browsing_prefs.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace safe_browsing {
 
 class SafeBrowsingPrefsTest : public ::testing::Test {
@@ skipping 15 matching lines @@
 
   void ResetPrefs(bool sber_reporting, bool scout_reporting, bool scout_group) {
     prefs_.SetBoolean(prefs::kSafeBrowsingExtendedReportingEnabled,
                       sber_reporting);
     prefs_.SetBoolean(prefs::kSafeBrowsingScoutReportingEnabled,
                       scout_reporting);
     prefs_.SetBoolean(prefs::kSafeBrowsingScoutGroupSelected, scout_group);
   }
 
   void ResetExperiments(bool can_show_scout, bool only_show_scout) {
-    std::vector<std::string> enabled_features;
-    std::vector<std::string> disabled_features;
+    std::vector<base::StringPiece> enabled_features;
+    std::vector<base::StringPiece> disabled_features;
 
     auto* target_vector =
         can_show_scout ? &enabled_features : &disabled_features;
     target_vector->push_back(kCanShowScoutOptIn.name);
 
     target_vector = only_show_scout ? &enabled_features : &disabled_features;
     target_vector->push_back(kOnlyShowScoutOptIn.name);
 
     feature_list_.reset(new base::test::ScopedFeatureList);
     feature_list_->InitFromCommandLine(
@@ skipping 396 matching lines @@
   EXPECT_EQ(SBER_LEVEL_OFF, GetExtendedReportingLevel(prefs_));
   // Turning on Scout gives us Scout level reporting
   ResetPrefs(/*sber=*/false, /*scout_reporting=*/true, /*scout_group=*/true);
   EXPECT_EQ(SBER_LEVEL_SCOUT, GetExtendedReportingLevel(prefs_));
   // .. and the legacy pref doesn't affect this.
   ResetPrefs(/*sber=*/true, /*scout_reporting=*/true, /*scout_group=*/true);
   EXPECT_EQ(SBER_LEVEL_SCOUT, GetExtendedReportingLevel(prefs_));
 }
 
 }  // namespace safe_browsing