Create thread-safe RendererExtensionRegistry from ExtensionSet

extensions/common/extension_set.h

Index: extensions/common/extension_set.h
diff --git a/extensions/common/extension_set.h b/extensions/common/extension_set.h
index c46163faef6a9ffb53e19c07160bd98ec17c531f..36e0fa390a1420d54b9df6dbd2d14c0d4dc89b10 100644
--- a/extensions/common/extension_set.h
+++ b/extensions/common/extension_set.h
@@ -139,6 +139,8 @@ class 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