Web MIDI: introduce MidiManager::Shutdown to shutdown gracefully

content/browser/media/midi_host_unittest.cc

Index: content/browser/media/midi_host_unittest.cc
diff --git a/content/browser/media/midi_host_unittest.cc b/content/browser/media/midi_host_unittest.cc
index bfe0c2f051bcaf41e12fac2500d3e3bc96412ebd..05f2dfef256e894e7d5daa9a38cb1ea637b66ed9 100644
--- a/content/browser/media/midi_host_unittest.cc
+++ b/content/browser/media/midi_host_unittest.cc
@@ -104,6 +104,10 @@ class MidiHostTest : public testing::Test {
         host_(new MidiHostForTesting(kRenderProcessId, &manager_)),
         data_(kNoteOn, kNoteOn + arraysize(kNoteOn)),
         port_id_(0) {}
+  ~MidiHostTest() override {
+    manager_.Shutdown();
+    RunLoopUntilIdle();
+  }
 
  protected:
   void AddOutputPort() {