Fix occassional hang in iOS 11 when calling VTDecompressionSessionInvalidate.

sdk/objc/Framework/Classes/Common/UIDevice+RTCDevice.mm

 /*
  *  Copyright 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 155 matching lines @@
 }
 
 + (double)currentDeviceSystemVersion {
   return [self currentDevice].systemVersion.doubleValue;
 }
 
 + (BOOL)isIOS9OrLater {
   return [self currentDeviceSystemVersion] >= 9.0;
 }
 
++ (BOOL)isIOS11OrLater {
+  return [self currentDeviceSystemVersion] >= 11.0;
+}
+
 @end