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.
+
+// FIXME: Implement forEach.

[Takashi Toyoshima, 2014/09/01 13:48:39]

Agreed to postpone forEach() this time. Since the IDL will be changed to have
'this' as a second argument later.

+// callback ForEachCallback = void(DOMString id, MIDIInput port);
+
+[
+    Iterable,
+    NoInterfaceObject,
+    GarbageCollected,
+] interface MIDIInputMap {
+    readonly attribute unsigned long size;
+    Iterator keys();

[Takashi Toyoshima, 2014/09/01 13:48:39]

Aha. Iterator can bring an argument of any type.
So, is it better to fix Web MIDI spec's IDL to return Iterator?
Now the spec uses MIDIKeyIterator and so on.
http://webaudio.github.io/web-midi-api/#dfn-midiinputmap

[yhirano, 2014/09/02 05:17:13]

"Iterator" is not a standard name (it is not exposed to Window). I hope the
WebIDL spec will invent proper names for iterator / itertable interfaces. Can we
leave them be and wait?

+    Iterator entries();
+    Iterator values();
+    // 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);
+};