IDL: Add forEach() method to iterable<>/maplike<>/setlike<> interfaces

Source/modules/webmidi/MIDIInputMap.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, MIDIInput port);
 
 [
     NoInterfaceObject,
     GarbageCollected,
 ] interface MIDIInputMap {
     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);
-    // FIXME: Implement forEach.
-    // void forEach (ForEachCallback callback);
     readonly maplike<DOMString, MIDIInput>;
 };