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

base/feature_list_unittest.cc

Index: base/feature_list_unittest.cc
diff --git a/base/feature_list_unittest.cc b/base/feature_list_unittest.cc
index fb3b320ae92f7022696f09da2736971ada564ab7..5fbd294dcf8102be299f0cfe429b99574f7e491f 100644
--- a/base/feature_list_unittest.cc
+++ b/base/feature_list_unittest.cc
@@ -14,6 +14,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/persistent_memory_allocator.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -33,7 +34,7 @@ struct Feature kFeatureOffByDefault {
 };
 
 std::string SortFeatureListString(const std::string& feature_list) {
-  std::vector<std::string> features =
+  std::vector<base::StringPiece> features =
       FeatureList::SplitFeatureListString(feature_list);
   std::sort(features.begin(), features.end());
   return JoinString(features, ",");