fix the crash in QTMonitorImpl::Start()

content/browser/device_monitor_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/device_monitor_mac.h"
 
 #import <QTKit/QTKit.h>
 
 #include "base/logging.h"
 
@@ skipping 22 matching lines @@
 DeviceMonitorMac::QTMonitorImpl::QTMonitorImpl(DeviceMonitorMac* monitor)
     : monitor_(monitor),
       number_audio_devices_(0),
       number_video_devices_(0),
       device_arrival_(nil),
       device_removal_(nil) {
   DCHECK(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
                     object:nil
                      queue:nil
                 usingBlock:^(NSNotification* notification) {
                     OnDeviceChanged();}];
 
   device_removal_ =
       [nc addObserverForName:QTCaptureDeviceWasDisconnectedNotification
@@ skipping 46 matching lines @@
   qt_monitor_->Stop();
 }
 
 void DeviceMonitorMac::NotifyDeviceChanged(
     base::SystemMonitor::DeviceType type) {
   // TODO(xians): Remove the global variable for SystemMonitor.
   base::SystemMonitor::Get()->ProcessDevicesChanged(type);
 }
 
 }  // namespace content