Create thread-safe RendererExtensionRegistry from ExtensionSet

extensions/renderer/dispatcher.h

Index: extensions/renderer/dispatcher.h
diff --git a/extensions/renderer/dispatcher.h b/extensions/renderer/dispatcher.h
index 87a45359386c2a0a78fdb6960be3e0b3e8ef479b..f8d6e26dc349c5686b459f601156e4c2d3f735e6 100644
--- a/extensions/renderer/dispatcher.h
+++ b/extensions/renderer/dispatcher.h
@@ -74,7 +74,7 @@ class Dispatcher : public content::RenderProcessObserver,
     return function_names_;
   }
 
-  const ExtensionSet* extensions() const { return &extensions_; }
+  const ExtensionSet* extensions() const;
 
   const ScriptContextSet& script_context_set() const {
     return *script_context_set_;
@@ -256,11 +256,6 @@ class Dispatcher : public content::RenderProcessObserver,
   // True if the IdleNotification timer should be set.
   bool set_idle_notifications_;
 
-  // Contains all loaded extensions.  This is essentially the renderer
-  // counterpart to ExtensionService in the browser. It contains information
-  // about all extensions currently loaded by the browser.
-  ExtensionSet extensions_;
-
   // The IDs of extensions that failed to load, mapped to the error message
   // generated on failure.
   std::map<std::string, std::string> extension_load_errors_;