Bug 1583996 - Unlock CubebDeviceEnumerator mutex during call to cubeb_enumerate_devices. r=achronop

Differential Revision: https://phabricator.services.mozilla.com/D48475

--HG--
extra : moz-landing-system : lando

dom/media/webrtc/CubebDeviceEnumerator.cpp

@@ -222,6 +222,8 @@ void CubebDeviceEnumerator::EnumerateAudioDevices(
 #else
   if (devices.IsEmpty() || manualInvalidation) {
     devices.Clear();
+
+    MutexAutoUnlock unlock(mMutex);
     GetDeviceCollection(devices, (aSide == Side::INPUT) ? CubebUtils::Input
                                                         : CubebUtils::Output);
   }