media/midi: Fix warnings found by clang chromium-style plugin on Windows.

media/midi/midi_manager_win.h

Index: media/midi/midi_manager_win.h
diff --git a/media/midi/midi_manager_win.h b/media/midi/midi_manager_win.h
index 73c1fe57972d3173153d4638fc79f9a0376d75dc..eef70eb55fd42df6ef6807c1423731a9c34a2bf0 100644
--- a/media/midi/midi_manager_win.h
+++ b/media/midi/midi_manager_win.h
@@ -41,8 +41,8 @@ class MidiServiceWin {
 
 class MidiManagerWin final : public MidiManager, public MidiServiceWinDelegate {
  public:
-  MidiManagerWin() {}
-  ~MidiManagerWin() override { midi_service_.reset(); }
+  MidiManagerWin();
+  ~MidiManagerWin() override;
 
   // MidiManager overrides:
   void StartInitialization() final;