Web MIDI: introduce MidiManager::Shutdown to shutdown gracefully

media/midi/midi_manager_usb_unittest.cc

Index: media/midi/midi_manager_usb_unittest.cc
diff --git a/media/midi/midi_manager_usb_unittest.cc b/media/midi/midi_manager_usb_unittest.cc
index 21dd2369262c5a0eea757532b3a1ccfdcdfe200f..bef5ab5a8a6080cc526b68728ef63b419a92d8bd 100644
--- a/media/midi/midi_manager_usb_unittest.cc
+++ b/media/midi/midi_manager_usb_unittest.cc
@@ -181,6 +181,7 @@ class MidiManagerUsbTest : public ::testing::Test {
     manager_.reset(new MidiManagerUsbForTesting(factory.Pass()));
   }
   ~MidiManagerUsbTest() override {
+    manager_->Shutdown();
     std::string leftover_logs = logger_.TakeLog();
     if (!leftover_logs.empty()) {
       ADD_FAILURE() << "Log should be empty: " << leftover_logs;