Make self.name in workers [Replaceable]

This is a follow-up to #2664, to
address the compatibility problem discovered in
https://bugzilla.mozilla.org/show_bug.cgi?id=1375457.

source

@@ -96659,7 +96659,7 @@ interface <dfn>WorkerGlobalScope</dfn> : <span>EventTarget</span> {
 
   <pre class="idl">[Global=(Worker,DedicatedWorker),Exposed=DedicatedWorker]
 interface <dfn>DedicatedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span> {
-  readonly attribute DOMString <span data-x="dom-DedicatedWorkerGlobalScope-name">name</span>;
+  [Replaceable] readonly attribute DOMString <span data-x="dom-DedicatedWorkerGlobalScope-name">name</span>;
 
   void <span data-x="dom-DedicatedWorkerGlobalScope-postMessage">postMessage</span>(any message, optional sequence&lt;<span data-x="idl-object">object</span>&gt; transfer = []);
 
@@ -96749,7 +96749,7 @@ interface <dfn>DedicatedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span>
 
   <pre class="idl">[Global=(Worker,SharedWorker),Exposed=SharedWorker]
 interface <dfn>SharedWorkerGlobalScope</dfn> : <span>WorkerGlobalScope</span> {
-  readonly attribute DOMString <span data-x="dom-SharedWorkerGlobalScope-name">name</span>;
+  [Replaceable] readonly attribute DOMString <span data-x="dom-SharedWorkerGlobalScope-name">name</span>;
 
   void <span data-x="dom-SharedWorkerGlobalScope-close">close</span>();