Expose Fetch API related classes to Window and WorkerGlobalScope.

Source/modules/serviceworkers/Headers.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // http://fetch.spec.whatwg.org/#headers-class
 typedef Dictionary OpenEndedDictionary;
 [
     Constructor,
     Constructor(Headers input),
     Constructor(OpenEndedDictionary input),
     Constructor(sequence<sequence<ByteString>> input),
     Iterable,
-    RuntimeEnabled=ServiceWorker,
-    Exposed=ServiceWorker,
+    Exposed(Window GlobalFetch, DedicatedWorker GlobalFetch, SharedWorker GlobalFetch, ServiceWorker ServiceWorker),
     RaisesException=Constructor,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Headers {
     [RaisesException] void append(ByteString name, ByteString value);
     [ImplementedAs=remove, RaisesException] void delete(ByteString key);
     [RaisesException] ByteString? get(ByteString key);
     [RaisesException] sequence<ByteString> getAll(ByteString name);
     [RaisesException] boolean has(ByteString key);
     [RaisesException] void set(ByteString key, ByteString value);
 };