Web MIDI: introduce MidiManager::Shutdown to shutdown gracefully

content/browser/media/midi_host.h

Index: content/browser/media/midi_host.h
diff --git a/content/browser/media/midi_host.h b/content/browser/media/midi_host.h
index c6ff3ea857dd75c0264d36378e1a22e546480262..a536ebec98da406197b9b0c676b9ae47a9e273b2 100644
--- a/content/browser/media/midi_host.h
+++ b/content/browser/media/midi_host.h
@@ -50,6 +50,7 @@ class CONTENT_EXPORT MidiHost : public BrowserMessageFilter,
                        size_t length,
                        double timestamp) override;
   void AccumulateMidiBytesSent(size_t n) override;
+  void Detach() override;
 
   // Start session to access MIDI hardware.
   void OnStartSession();
@@ -70,7 +71,7 @@ class CONTENT_EXPORT MidiHost : public BrowserMessageFilter,
   friend class BrowserThread;
 
   // Returns true if |data| fulfills the requirements of MidiOutput.send API
-  // defined in the WebMIDI spec.
+  // defined in the Web MIDI spec.
   // - |data| must be any number of complete MIDI messages (data abbreviation
   //    called "running status" is disallowed).
   // - 1-byte MIDI realtime messages can be placed at any position of |data|.
@@ -90,7 +91,7 @@ class CONTENT_EXPORT MidiHost : public BrowserMessageFilter,
   // does not support MIDI.  If not supported then a call to
   // OnRequestAccess() will always refuse access and a call to
   // OnSendData() will do nothing.
-  media::midi::MidiManager* const midi_manager_;
+  media::midi::MidiManager* midi_manager_;
 
   // Buffers where data sent from each MIDI input port is stored.
   ScopedVector<media::midi::MidiMessageQueue> received_messages_queues_;