[WebMIDI] Add "use-android-midi-api" flag for Android M+ devices.

media/midi/midi_switches.h

+// Copyright 2015 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.
+
+// Defines all the "midi" command-line switches.
+
+#ifndef MEDIA_MIDI_MIDI_SWITCHES_H_
+#define MEDIA_MIDI_MIDI_SWITCHES_H_
+
+#include "build/build_config.h"
+#include "media/midi/midi_export.h"
+
+namespace switches {
+
+#if defined(OS_ANDROID)
+MIDI_EXPORT extern const char kUseAndroidMidiApi[];
+#endif
+
+}  // namespace switches
+
+#endif  // MEDIA_MIDI_MIDI_SWITCHES_H_