Rework some UserScriptLoader logic in preparation to removing UserScript copy.

chrome/browser/extensions/shared_user_script_master.h

Index: chrome/browser/extensions/shared_user_script_master.h
diff --git a/chrome/browser/extensions/shared_user_script_master.h b/chrome/browser/extensions/shared_user_script_master.h
index b697829e98a6f82889a89ecfece5227373eb0bc9..5fb32d413df7f7ba8bfa18eb8fd6ce32285aef5d 100644
--- a/chrome/browser/extensions/shared_user_script_master.h
+++ b/chrome/browser/extensions/shared_user_script_master.h
@@ -45,7 +45,7 @@ class SharedUserScriptMaster : public ExtensionRegistryObserver {
 
   // Gets an extension's scripts' metadata; i.e., gets a list of UserScript
   // objects that contains script info, but not the contents of the scripts.
-  const std::set<UserScript> GetScriptsMetadata(const Extension* extension);
+  const UserScriptList GetScriptsMetadata(const Extension* extension);
 
   // Script loader that handles loading contents of scripts into shared memory
   // and notifying renderers of scripts in shared memory.