Allow SynchronizedFrameDecodeScheduler::Stop to be run multiple times

Stop being called twice can happen in tests since the VideoReceiveStream
destructor calls Stop so any test calling Stop may invoke it twice. This
is a general problem that all things that the VideoReceiveStream have to
able to be stopped multiple times.

Bug: b/270932185
Change-Id: Ic25810d5ab73e8a07cf3b16685c578f4c0aa7fbd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/295580
Auto-Submit: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39452}

video/decode_synchronizer.cc

diff --git a/video/decode_synchronizer.cc b/video/decode_synchronizer.cc
index 7d4da3d..e676acf 100644
--- a/video/decode_synchronizer.cc
+++ b/video/decode_synchronizer.cc
@@ -66,6 +66,7 @@
 
 DecodeSynchronizer::ScheduledFrame
 DecodeSynchronizer::SynchronizedFrameDecodeScheduler::ReleaseNextFrame() {
+  RTC_DCHECK(!stopped_);
   RTC_DCHECK(next_frame_);
   auto res = std::move(*next_frame_);
   next_frame_.reset();
@@ -82,6 +83,7 @@
     uint32_t rtp,
     FrameDecodeTiming::FrameSchedule schedule,
     FrameReleaseCallback cb) {
+  RTC_DCHECK(!stopped_);
   RTC_DCHECK(!next_frame_) << "Can not schedule two frames at once.";
   next_frame_ = ScheduledFrame(rtp, std::move(schedule), std::move(cb));
   sync_->OnFrameScheduled(this);
@@ -92,6 +94,9 @@
 }
 
 void DecodeSynchronizer::SynchronizedFrameDecodeScheduler::Stop() {
+  if (stopped_) {
+    return;
+  }
   CancelOutstanding();
   stopped_ = true;
   sync_->RemoveFrameScheduler(this);
@@ -107,7 +112,7 @@
 
 DecodeSynchronizer::~DecodeSynchronizer() {
   RTC_DCHECK_RUN_ON(worker_queue_);
-  RTC_DCHECK(schedulers_.empty());
+  RTC_CHECK(schedulers_.empty());
 }
 
 std::unique_ptr<FrameDecodeScheduler>