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

base/feature_list.h

 // Copyright 2015 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.
 
 #ifndef BASE_FEATURE_LIST_H_
 #define BASE_FEATURE_LIST_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 138 matching lines @@
   // Returns whether the given |feature| is enabled. Must only be called after
   // the singleton instance has been registered via SetInstance(). Additionally,
   // a feature with a given name must only have a single corresponding Feature
   // struct, which is checked in builds with DCHECKs enabled.
   static bool IsEnabled(const Feature& feature);
 
   // Returns the field trial associated with the given |feature|. Must only be
   // called after the singleton instance has been registered via SetInstance().
   static FieldTrial* GetFieldTrial(const Feature& feature);
 
-  // Splits a comma-separated string containing feature names into a vector.
-  static std::vector<std::string> SplitFeatureListString(
-      const std::string& input);
+  // Splits a comma-separated string containing feature names into a vector. The
+  // resulting pieces point to parts of |input|.
+  static std::vector<base::StringPiece> SplitFeatureListString(
+      base::StringPiece input);
 
   // Initializes and sets an instance of FeatureList with feature overrides via
   // command-line flags |enable_features| and |disable_features| if one has not
   // already been set from command-line flags. Returns true if an instance did
   // not previously exist. See InitializeFromCommandLine() for more details
   // about |enable_features| and |disable_features| parameters.
   static bool InitializeInstance(const std::string& enable_features,
                                  const std::string& disable_features);
 
   // Returns the singleton instance of FeatureList. Will return null until an
@@ skipping 104 matching lines @@
 
   // Whether this object has been initialized from command line.
   bool initialized_from_command_line_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(FeatureList);
 };
 
 }  // namespace base
 
 #endif  // BASE_FEATURE_LIST_H_