MIDI{Input, Output}Map[Symbol.iterator]() should return entries.

ES6 Map returns `entries' method when [@@iterator] property is required[1].
MIDIInputMap and MIDIOutputMap should do the same.

1: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype-@@iterator

BUG=341423
R=toyoshim@chromium.org

Review URL: https://codereview.chromium.org/555683003

git-svn-id: svn://svn.chromium.org/blink/trunk@181616 bbb929c8-8fbe-4397-9dbb-9b2b20218538