Remove FrameBuffer::ReturnReason

This was a remenant leftover from a previous design, which was no longer
valid after the switch to TaskQueues. ReturnReason::kStopped was not
used at all, and so Timeout or FrameFound can be inferred from whether
the frame is null or not.

Bug: webrtc:13343, webrtc:13346
Change-Id: Ib0f847b1e1192e32ea11208e48f5a3892703521e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/239651
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35490}
diff --git a/modules/video_coding/frame_buffer2_unittest.cc b/modules/video_coding/frame_buffer2_unittest.cc
index e498afd..be838c7 100644
--- a/modules/video_coding/frame_buffer2_unittest.cc
+++ b/modules/video_coding/frame_buffer2_unittest.cc
@@ -199,14 +199,10 @@
 
   void ExtractFrame(int64_t max_wait_time = 0, bool keyframe_required = false) {
     time_task_queue_.PostTask([this, max_wait_time, keyframe_required]() {
-      buffer_->NextFrame(
-          max_wait_time, keyframe_required, &time_task_queue_,
-          [this](std::unique_ptr<EncodedFrame> frame,
-                 video_coding::FrameBuffer::ReturnReason reason) {
-            if (reason != FrameBuffer::ReturnReason::kStopped) {
-              frames_.emplace_back(std::move(frame));
-            }
-          });
+      buffer_->NextFrame(max_wait_time, keyframe_required, &time_task_queue_,
+                         [this](std::unique_ptr<EncodedFrame> frame) {
+                           frames_.emplace_back(std::move(frame));
+                         });
     });
     if (max_wait_time == 0) {
       time_controller_.AdvanceTime(TimeDelta::Millis(0));