[Extensions] Correct a class name in a comment in UserScriptInjector

"The associated user script. Owned by the UserScriptInjector that
created this object."

^Should be UserScriptSet.

Bug: NONE
Change-Id: I5a080c33b74a22b8af3b824234a0a89aff8fa9ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537410Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827397}

extensions/renderer/user_script_injector.h

@@ -68,8 +68,8 @@ class UserScriptInjector : public ScriptInjector,
   void OnWillNotInject(InjectFailureReason reason,
                        content::RenderFrame* render_frame) override;
 
-  // The associated user script. Owned by the UserScriptInjector that created
-  // this object.
+  // The associated user script. Owned by the UserScriptSet that created this
+  // object.
   const UserScript* script_;
 
   // The UserScriptSet that eventually owns the UserScript this