Web MIDI: Add CHECK()s to ensure MidiManager owned threads are terminated

media/midi/midi_manager_alsa.cc

Index: media/midi/midi_manager_alsa.cc
diff --git a/media/midi/midi_manager_alsa.cc b/media/midi/midi_manager_alsa.cc
index 9dc0582e6a41cccff48e992a0d6f8852cff7e25f..dbbd5edbbffbf9ea18c375f710f071a577f90c1d 100644
--- a/media/midi/midi_manager_alsa.cc
+++ b/media/midi/midi_manager_alsa.cc
@@ -165,6 +165,9 @@ MidiManagerAlsa::~MidiManagerAlsa() {
   DCHECK(!decoder_);
   DCHECK(!udev_);
   DCHECK(!udev_monitor_);
+
+  CHECK(!send_thread_.IsRunning());

[Adam Goode, 2016/07/21 15:40:30]

I don't think you can be sure of the result of IsRunning() if called from a
different thread than calls Start().

[Takashi Toyoshima, 2016/07/21 18:25:24]

IsRunning() is safe to be called from any thread.
Of course, Start() and Stop() could be called any time. But in our case, these
are called only on the IO thread in StartInitialization() and Finalize(), and
the IO thread stops before destructing MidiManager and BrowserMainLoop.

So this check works in practice.

+  CHECK(!event_thread_.IsRunning());
 }
 
 void MidiManagerAlsa::StartInitialization() {