fix the crash in QTMonitorImpl::Start()

content/browser/device_monitor_mac.mm

Index: content/browser/device_monitor_mac.mm
diff --git a/content/browser/device_monitor_mac.mm b/content/browser/device_monitor_mac.mm
index bdf97baef78e97d7c418feaef4524d07e581f1d2..7b12e91cbdf4ed8b57fc020d3b75c0579240d0ef 100644
--- a/content/browser/device_monitor_mac.mm
+++ b/content/browser/device_monitor_mac.mm
@@ -40,17 +40,6 @@ DeviceMonitorMac::QTMonitorImpl::QTMonitorImpl(DeviceMonitorMac* monitor)
 }
 
 void DeviceMonitorMac::QTMonitorImpl::Start() {
-  NSArray* devices = [QTCaptureDevice inputDevices];
-  for (QTCaptureDevice* device in devices) {
-    if ([device hasMediaType:QTMediaTypeVideo] ||
-        [device hasMediaType:QTMediaTypeMuxed])
-      ++number_video_devices_;
-
-    if ([device hasMediaType:QTMediaTypeSound] ||
-        [device hasMediaType:QTMediaTypeMuxed])
-      ++number_audio_devices_;
-  }
-
   NSNotificationCenter* nc = [NSNotificationCenter defaultCenter];
   device_arrival_ =
     [nc addObserverForName:QTCaptureDeviceWasConnectedNotification