Prevent a race condition with MidiHost IPC sending.

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 671bd3c6543ccf40b4e1a1b3e1d46b1cdb76f8ea..e0f60f3a7f16e2bff61f67f9740c3245df61ef34 100644
--- a/content/browser/media/midi_host.h
+++ b/content/browser/media/midi_host.h
@@ -38,8 +38,10 @@ class CONTENT_EXPORT MidiHost : public BrowserMessageFilter,
   MidiHost(int renderer_process_id, media::midi::MidiManager* midi_manager);
 
   // BrowserMessageFilter implementation.
+  void OnChannelClosing() override;
   void OnDestruct() const override;
   bool OnMessageReceived(const IPC::Message& message) override;
+  bool Send(IPC::Message* msg) override;
 
   // MidiManagerClient implementation.
   void CompleteStartSession(media::midi::Result result) override;
@@ -120,6 +122,12 @@ class CONTENT_EXPORT MidiHost : public BrowserMessageFilter,
   // Protects access to |output_port_count_|.
   base::Lock output_port_count_lock_;
 
+  // Represents whether if the IPC channel is about to close.
+  bool is_channel_closing_;
+
+  // Protects access to |is_channel_closing_|.
+  base::Lock is_channel_closing_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(MidiHost);
 };