Update VideoFrameScheduler API and add clockless and testing implementations.

media/filters/video_frame_scheduler_impl.cc

Index: media/filters/video_frame_scheduler_impl.cc
diff --git a/media/filters/video_frame_scheduler_impl.cc b/media/filters/video_frame_scheduler_impl.cc
index a50516446afeb7a7cc57e6f3ed7ac4bab1662906..ee06bb1cd96b7dbc70517ec061b2c1b572315f24 100644
--- a/media/filters/video_frame_scheduler_impl.cc
+++ b/media/filters/video_frame_scheduler_impl.cc
@@ -34,11 +34,8 @@ void VideoFrameSchedulerImpl::ScheduleVideoFrame(
 }
 
 void VideoFrameSchedulerImpl::Reset() {
-  DCHECK(task_runner_->BelongsToCurrentThread());
-  while (!pending_frames_.empty()) {
-    pending_frames_.top().done_cb.Run(pending_frames_.top().frame, RESET);
-    pending_frames_.pop();
-  }
+  pending_frames_ = PendingFrameQueue();
+  timer_.Stop();
 }
 
 void VideoFrameSchedulerImpl::SetTickClockForTesting(