Remove a thread checker for a decoder thread from IncomingVideoStream.
The specific decoder thread may vary when using VideoToolbox.

BUG=webrtc:6051
TBR=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2100083002 .

Cr-Original-Commit-Position: refs/heads/master@{#13292}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7893e9a267769940d6fd4276196208ee91d7afbf
diff --git a/common_video/include/incoming_video_stream.h b/common_video/include/incoming_video_stream.h
index b551d45..a23ac05 100644
--- a/common_video/include/incoming_video_stream.h
+++ b/common_video/include/incoming_video_stream.h
@@ -42,7 +42,6 @@
 
   rtc::ThreadChecker main_thread_checker_;
   rtc::ThreadChecker render_thread_checker_;
-  rtc::ThreadChecker decoder_thread_checker_;
 
   rtc::CriticalSection buffer_critsect_;
   rtc::PlatformThread incoming_render_thread_;
diff --git a/common_video/incoming_video_stream.cc b/common_video/incoming_video_stream.cc
index 8deca0f..f5a52fc 100644
--- a/common_video/incoming_video_stream.cc
+++ b/common_video/incoming_video_stream.cc
@@ -29,7 +29,6 @@
   RTC_DCHECK(external_callback_);
 
   render_thread_checker_.DetachFromThread();
-  decoder_thread_checker_.DetachFromThread();
 
   incoming_render_thread_.Start();
   incoming_render_thread_.SetPriority(rtc::kRealtimePriority);
@@ -50,7 +49,8 @@
 }
 
 void IncomingVideoStream::OnFrame(const VideoFrame& video_frame) {
-  RTC_DCHECK_RUN_ON(&decoder_thread_checker_);
+  // Most of the time we'll be on a decoder thread here, but when using
+  // VideoToolbox on iOS, we'll get called on a thread from a thread pool.
 
   // Hand over or insert frame.
   rtc::CritScope csB(&buffer_critsect_);