Mac AVFoundation: check -CrAVCaptureDevice::observationInfo before removing observers.

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 <set>
 
@@ skipping 360 matching lines @@
               options:0
               context:device];
   monitoredDevices_.insert(device);
 }
 
 - (void)stopObserving:(CrAVCaptureDevice*)device {
   DCHECK(device != nil);
   std::set<CrAVCaptureDevice*>::iterator found =
       std::find(monitoredDevices_.begin(), monitoredDevices_.end(), device);
   DCHECK(found != monitoredDevices_.end());
-  [device removeObserver:self
-              forKeyPath:@"suspended"];
-  [device removeObserver:self
-              forKeyPath:@"connected"];
+  // Every so seldom, |device| might be gone when getting here, in that case
+  // removing the observer causes a crash. Try to avoid it by checking sanity of
+  // the |device| via its -observationInfo. http://crbug.com/371271.
+  if ([device observationInfo]) {
+    [device removeObserver:self
+                forKeyPath:@"suspended"];
+    [device removeObserver:self
+                forKeyPath:@"connected"];
+  }
   monitoredDevices_.erase(found);
 }
 
 - (void)observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context {
   if ([keyPath isEqual:@"suspended"])
     receiver_->OnDeviceChanged();
   if ([keyPath isEqual:@"connected"])
@@ skipping 27 matching lines @@
 }
 
 void DeviceMonitorMac::NotifyDeviceChanged(
     base::SystemMonitor::DeviceType type) {
   DCHECK(thread_checker_.CalledOnValidThread());
   // TODO(xians): Remove the global variable for SystemMonitor.
   base::SystemMonitor::Get()->ProcessDevicesChanged(type);
 }
 
 }  // namespace content