Web MIDI: add dynamic MidiManager instantiation support for Linux

media/midi/midi_manager.h

 // Copyright (c) 2013 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.
 
 #ifndef MEDIA_MIDI_MIDI_MANAGER_H_
 #define MEDIA_MIDI_MIDI_MANAGER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 55 matching lines @@
   // This happens as a result of the client having previously called
   // MidiManager::DispatchSendMidiData().
   virtual void AccumulateMidiBytesSent(size_t n) = 0;
 
   // Detach() is called when MidiManager is going to shutdown immediately.
   // Client should not touch MidiManager instance after Detach() is called.
   virtual void Detach() = 0;
 };
 
 // Manages access to all MIDI hardware.
+// *** Note ***: If dynamic instantiation feature is enabled, all MidiManager
+// methods will be called on Chrome_IOThread. See comments on Shutdown() too.
 class MIDI_EXPORT MidiManager {
  public:
   static const size_t kMaxPendingClientCount = 128;
 
   MidiManager();
   virtual ~MidiManager();
 
   // The constructor and the destructor will be called on the CrBrowserMain
   // thread.
   static MidiManager* Create();
 
   // Called on the CrBrowserMain thread to notify the Chrome_IOThread will stop
   // and the instance will be destructed on the CrBrowserMain thread soon.
+  // *** Note ***: If dynamic instantiation feature is enabled, MidiService call
+  // this on Chrome_IOThread and ShutdownOnSessionThread() will be called
+  // synchronously so that MidiService can destruct MidiManager synchronously.
   void Shutdown();
 
   // A client calls StartSession() to receive and send MIDI data.
   // If the session is ready to start, the MIDI system is lazily initialized
   // and the client is registered to receive MIDI data.
   // CompleteStartSession() is called with mojom::Result::OK if the session is
   // started. Otherwise CompleteStartSession() is called with a proper
   // mojom::Result code.
   // StartSession() and EndSession() can be called on the Chrome_IOThread.
   // CompleteStartSession() will be invoked on the same Chrome_IOThread.
@@ skipping 116 matching lines @@
   // |session_thread_runner_|, |initialization_state_|, |finalize_|, |result_|,
   // |input_ports_| and |output_ports_|.
   base::Lock lock_;
 
   DISALLOW_COPY_AND_ASSIGN(MidiManager);
 };
 
 }  // namespace midi
 
 #endif  // MEDIA_MIDI_MIDI_MANAGER_H_