Fix occassional hang in iOS 11 when calling VTDecompressionSessionInvalidate.

sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm

 /*
  *  Copyright (c) 2015 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.
  *
  */
 
 #import "WebRTC/RTCVideoCodecH264.h"
 
 #import <VideoToolbox/VideoToolbox.h>
 
 #include "modules/video_coding/include/video_error_codes.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/timeutils.h"
 #include "sdk/objc/Framework/Classes/VideoToolbox/nalu_rewriter.h"
 
 #import "WebRTC/RTCVideoFrame.h"
 #import "WebRTC/RTCVideoFrameBuffer.h"
 #import "helpers.h"
 
 #if defined(WEBRTC_IOS)
 #import "Common/RTCUIApplicationStatusObserver.h"
+#import "WebRTC/UIDevice+RTCDevice.h"
 #endif
 
 // Struct that we pass to the decoder per frame to decode. We receive it again
 // in the decoder callback.
 struct RTCFrameDecodeParams {
   RTCFrameDecodeParams(RTCVideoDecoderCallback cb, int64_t ts) : callback(cb), timestamp(ts) {}
   RTCVideoDecoderCallback callback;
   int64_t timestamp;
 };
 
@@ skipping 191 matching lines @@
 
 - (void)configureDecompressionSession {
   RTC_DCHECK(_decompressionSession);
 #if defined(WEBRTC_IOS)
   VTSessionSetProperty(_decompressionSession, kVTDecompressionPropertyKey_RealTime, kCFBooleanTrue);
 #endif
 }
 
 - (void)destroyDecompressionSession {
   if (_decompressionSession) {
-    VTDecompressionSessionInvalidate(_decompressionSession);
+#if defined(WEBRTC_IOS)
+    if ([UIDevice isIOS11OrLater]) {
+      VTDecompressionSessionWaitForAsynchronousFrames(_decompressionSession);
+    } else {
+#endif
+      VTDecompressionSessionInvalidate(_decompressionSession);
+#if defined(WEBRTC_IOS)
+    }
+#endif
     CFRelease(_decompressionSession);
     _decompressionSession = nullptr;
   }
 }
 
 - (void)setVideoFormat:(CMVideoFormatDescriptionRef)videoFormat {
   if (_videoFormat == videoFormat) {
     return;
   }
   if (_videoFormat) {
     CFRelease(_videoFormat);
   }
   _videoFormat = videoFormat;
   if (_videoFormat) {
     CFRetain(_videoFormat);
   }
 }
 
 - (NSString *)implementationName {
   return @"VideoToolbox";
 }
 
 @end