[Coverity] Fix pass-by-val/pass-by-ref

chrome/common/extensions/permissions/permission_set.cc

 // Copyright (c) 2012 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 "chrome/common/extensions/permissions/permission_set.h"
 
 #include <algorithm>
 #include <iterator>
 #include <string>
 
 #include "chrome/common/extensions/extension.h"
 #include "chrome/common/extensions/permissions/permissions_info.h"
 #include "chrome/common/extensions/url_pattern.h"
 #include "chrome/common/extensions/url_pattern_set.h"
 #include "content/public/common/url_constants.h"
 #include "grit/generated_resources.h"
 #include "net/base/registry_controlled_domain.h"
 #include "ui/base/l10n/l10n_util.h"
 
 namespace {
 
 // Helper for GetDistinctHosts(): com > net > org > everything else.
-bool RcdBetterThan(std::string a, std::string b) {
+bool RcdBetterThan(const std::string& a, const std::string& b) {
   if (a == b)
     return false;
   if (a == "com")
     return true;
   if (a == "net")
     return b != "com";
   if (a == "org")
     return b != "com" && b != "net";
   return false;
 }
@@ skipping 544 matching lines @@
   std::set<std::string> new_hosts_only;
 
   std::set_difference(new_hosts_set.begin(), new_hosts_set.end(),
                       old_hosts_set.begin(), old_hosts_set.end(),
                       std::inserter(new_hosts_only, new_hosts_only.begin()));
 
   return !new_hosts_only.empty();
 }
 
 }  // namespace extensions