Web MIDI: add dynamic MidiManager instantiation support for Linux

media/midi/midi_switches.cc

 // 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.
 
 #include "build/build_config.h"
 #include "media/midi/midi_switches.h"
 
 namespace midi {
 namespace features {
 
 #if defined(OS_ANDROID)
 const base::Feature kMidiManagerAndroid{"MidiManagerAndroid",
                                         base::FEATURE_DISABLED_BY_DEFAULT};
 #endif
 
 #if defined(OS_WIN)
 const base::Feature kMidiManagerWinrt{"MidiManagerWinrt",
                                       base::FEATURE_DISABLED_BY_DEFAULT};
 #endif
 
+const base::Feature kMidiManagerDynamicInstantiation{
+    "MidiManagerDynamicInstantiation", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace midi