Add Maplike<> utility mixin class for implementing maplike interfaces

Source/modules/encryptedmedia/MediaKeyStatusMap.idl

 // Copyright 2015 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.
 
 // https://w3c.github.io/encrypted-media/#idl-def-MediaKeyStatusMap
 
 enum MediaKeyStatus {
     "usable",
     "expired",
     "output-not-allowed",
     "internal-error"
 };
 
 [
     GarbageCollected,
 ] interface MediaKeyStatusMap {
     readonly attribute unsigned long size;
-
-    // This function returns undefined if |!this->has(key)|.
-    [TreatReturnedNullStringAs=Undefined] MediaKeyStatus get(BufferSource key);
-    boolean has(BufferSource key);
-
     readonly maplike <BufferSource, MediaKeyStatus>;
 };