Web MIDI: Improve device change monitornig on Windows

Current implementation monitors DBT_DEVICEARRIVAL mesage, but that
is not a right message to monitor device changes since actual device
list that OS holds will be updated a little later. Instead, monitor
DBT_DEVNODES_CHANGES that is delivered after the list being updated.

In Chrome, SystemMessageWindowWin invokes base::SystemMonitor's observer
with DEVTYPE_UNKNOWN for DBT_DEVNODES_CHANGED, and the SystemMonitor
provides a way to observe the event on the registered thread.
Using the monitor must be the right implementation.

BUG=472980

Review URL: https://codereview.chromium.org/1056333002

Cr-Commit-Position: refs/heads/master@{#324031}

media/midi/midi_manager_unittest.cc

@@ -12,6 +12,7 @@
 #include "base/memory/scoped_vector.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/system_monitor/system_monitor.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace media {
@@ -245,6 +246,9 @@ TEST_F(MidiManagerTest, AbortSession) {
 }
 
 TEST_F(MidiManagerTest, CreateMidiManager) {
+  // SystemMonitor is needed on Windows.
+  base::SystemMonitor system_monitor;
+
   scoped_ptr<FakeMidiManagerClient> client;
   client.reset(new FakeMidiManagerClient);