Use container::back() and container::pop_back().

extensions/common/features/base_feature_provider.cc

 // Copyright 2014 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 "extensions/common/features/base_feature_provider.h"
 
 #include <stddef.h>
 
 #include <stack>
 #include <utility>
@@ skipping 162 matching lines @@
 }
 
 // Children of a given API are named starting with parent.name()+".", which
 // means they'll be contiguous in the features_ std::map.
 std::vector<Feature*> BaseFeatureProvider::GetChildren(const Feature& parent)
     const {
   std::string prefix = parent.name() + ".";
   const FeatureMap::const_iterator first_child = features_.lower_bound(prefix);
 
   // All children have names before (parent.name() + ('.'+1)).
-  ++prefix[prefix.size() - 1];
+  ++prefix.back();
   const FeatureMap::const_iterator after_children =
       features_.lower_bound(prefix);
 
   std::vector<Feature*> result;
   result.reserve(std::distance(first_child, after_children));
   for (FeatureMap::const_iterator it = first_child; it != after_children;
        ++it) {
     result.push_back(it->second.get());
   }
   return result;
 }
 
 }  // namespace extensions