Web MIDI: add dynamic MidiManager instantiation support for Linux

media/midi/midi_manager_alsa.h

Index: media/midi/midi_manager_alsa.h
diff --git a/media/midi/midi_manager_alsa.h b/media/midi/midi_manager_alsa.h
index 0ff58a76c10dcf3805e0ed2e1093a79caa2313bc..1b77ab1c37e2a4f648807ad64c02e3eea4e3c79f 100644
--- a/media/midi/midi_manager_alsa.h
+++ b/media/midi/midi_manager_alsa.h
@@ -373,10 +373,12 @@ class MIDI_EXPORT MidiManagerAlsa final : public MidiManager {
       std::unique_ptr<snd_midi_event_t, SndMidiEventDeleter>;
 
   // An internal callback that runs on MidiSendThread.
-  void SendMidiData(uint32_t port_index, const std::vector<uint8_t>& data);
+  void SendMidiData(int instance_id,
+                    MidiManagerClient* client,
+                    uint32_t port_index,
+                    const std::vector<uint8_t>& data);
 
-  void ScheduleEventLoop();
-  void EventLoop();
+  void EventLoop(int instance_id);
   void ProcessSingleEvent(snd_seq_event_t* event, double timestamp);
   void ProcessClientStartEvent(int client_id);
   void ProcessPortStartEvent(const snd_seq_addr_t& addr);
@@ -447,10 +449,7 @@ class MIDI_EXPORT MidiManagerAlsa final : public MidiManager {
   device::ScopedUdevPtr udev_;
   device::ScopedUdevMonitorPtr udev_monitor_;
 
-  // Threads for sending and receiving. These are initialized in the
-  // constructor, but are started at the end of StartInitialization.
-  base::Thread event_thread_;
-  base::Thread send_thread_;
+  int instance_id_;
 
   DISALLOW_COPY_AND_ASSIGN(MidiManagerAlsa);
 };