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

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 5491aa640eec7d595651fc86290aca82eaee63f8..ae720cbdd9686017b2b934438eb6af6e00e1b8f5 100644
--- a/content/browser/device_monitor_mac.mm
+++ b/content/browser/device_monitor_mac.mm
@@ -378,10 +378,15 @@ void AVFoundationMonitorImpl::StartObserverOnDeviceThread() {
   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);
 }