Introduce MIDIInputMap and MIDIOutputMap.

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.
+
+// callback ForEachCallback = void(DOMString id, MIDIInput port);
+
+[
+    Iterable,
+    NoInterfaceObject,
+    GarbageCollected,
+] interface MIDIInputMap {
+    readonly attribute unsigned long size;
+    Iterator keys();
+    Iterator entries();
+    Iterator values();
+    // This function returns undefined if |!this->has(id)|.
+    [ImplementedAs=getPublic, CallWith=ScriptState] any get(DOMString id);
+    boolean has(DOMString key);
+//    void forEach (ForEachCallback callback);
+};