Web MIDI: add dynamic MidiManager instantiation support for Linux

media/midi/midi_manager.h

Index: media/midi/midi_manager.h
diff --git a/media/midi/midi_manager.h b/media/midi/midi_manager.h
index 27e24f92cd9c86937622e5f311977d5e59be2252..826cd8132497a6c99b482c0dfd58cc96e78c49cd 100644
--- a/media/midi/midi_manager.h
+++ b/media/midi/midi_manager.h
@@ -73,6 +73,8 @@ class MIDI_EXPORT MidiManagerClient {
 };
 
 // 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;
@@ -86,6 +88,9 @@ class MIDI_EXPORT MidiManager {
 
   // 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.