[ServiceWorker] Implement Headers class.

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
+[
+    Constructor,
+    Constructor(Headers input),
+    Constructor(Dictionary input),
+    RuntimeEnabled=ServiceWorker,
+    Exposed=ServiceWorker,
+    RaisesException=Constructor
+] interface Headers {
+    [RaisesException] void append(ByteString name, ByteString value);
+    [ImplementedAs=remove, RaisesException] void delete(ByteString key);
+    [TreatReturnedNullStringAs=Null, RaisesException] ByteString? get(ByteString key);
+    [RaisesException] sequence<ByteString> getAll(ByteString name);
+    [RaisesException] boolean has(ByteString key);
+    [RaisesException] void set(ByteString key, ByteString value);
+
+    // The following are not defined in the spec yet.
+    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=26102
+    readonly attribute unsigned long size;
+    void forEach(HeadersForEachCallback callback, optional any thisArg);
+};