Add Maplike<> utility mixin class for implementing maplike interfaces

Source/modules/webmidi/MIDIOutputMap.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.
 
 // FIXME: Implement forEach.
 // callback ForEachCallback = void(DOMString id, MIDIOutput port);
 
 [
     NoInterfaceObject,
     GarbageCollected,
 ] interface MIDIOutputMap {
     readonly attribute unsigned long size;
-    // This function returns undefined if |!this->has(id)|.
-    [ImplementedAs=getForBinding, CallWith=ScriptState] any get(DOMString id);
-    boolean has(DOMString key);
     readonly maplike<DOMString, MIDIOutput>;
 };