Move Fetch API releted code to modules/fetch.

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,
-    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);
-};