Create thread-safe RendererExtensionRegistry from ExtensionSet

extensions/common/extension_set.h

 // Copyright 2013 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 EXTENSIONS_COMMON_EXTENSION_SET_H_
 #define EXTENSIONS_COMMON_EXTENSION_SET_H_
 
 #include <iterator>
 #include <map>
 #include <string>
@@ skipping 121 matching lines @@
   void set_modification_callback(
       const ModificationCallback& modification_callback) {
     modification_callback_ = modification_callback;
   }
 
  private:
   FRIEND_TEST_ALL_PREFIXES(ExtensionSetTest, ExtensionSet);
 
   ExtensionMap extensions_;
 
+  mutable base::Lock runtime_lock_;
+
   // If non-null, called with the extension ids in this set after a modification
   // occurred. This is not called on Clear() which is typically used when
   // discarding the set (e.g., on shutdown) and we do not want to track that as
   // a real modification.
   ModificationCallback modification_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionSet);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_EXTENSION_SET_H_