Web MIDI: introduce MidiManager::Shutdown to shutdown gracefully

media/midi/midi_manager_unittest.cc

Index: media/midi/midi_manager_unittest.cc
diff --git a/media/midi/midi_manager_unittest.cc b/media/midi/midi_manager_unittest.cc
index 2b56780e22e793fea044a223692ba74ad13b23f0..da007e3fba182febd956f9426bfcea367c218ea9 100644
--- a/media/midi/midi_manager_unittest.cc
+++ b/media/midi/midi_manager_unittest.cc
@@ -100,7 +100,9 @@ class MidiManagerTest : public ::testing::Test {
   MidiManagerTest()
       : manager_(new FakeMidiManager),
         message_loop_(new base::MessageLoop) {}
-  ~MidiManagerTest() override {}
+  ~MidiManagerTest() override {
+    manager_->Shutdown();
+  }
 
  protected:
   void StartTheFirstSession(FakeMidiManagerClient* client) {
@@ -267,6 +269,8 @@ TEST_F(MidiManagerTest, CreateMidiManager) {
 #else
   EXPECT_EQ(Result::OK, result);
 #endif
+
+  manager->Shutdown();
 }
 
 }  // namespace