Web MIDI: add dynamic MidiManager instantiation support for Linux

media/midi/midi_switches.cc

Index: media/midi/midi_switches.cc
diff --git a/media/midi/midi_switches.cc b/media/midi/midi_switches.cc
index 236a13a3fde51fdda04bb5b8f3b18f65a188f97e..96e7974b9b9cef034d8735bd49076a3174fbb81f 100644
--- a/media/midi/midi_switches.cc
+++ b/media/midi/midi_switches.cc
@@ -18,5 +18,8 @@ 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