media/gpu/v4l2vea: Enqueue V4L2 output buffers after using them.

In CL:1824303, we changed the flow of using V4L2 output buffers. At
that CL, we forgot to enqueue V4L2 output buffers after we copy the
content to bitstream buffers and free them.
This CL fixes this issue by calling Enqueue() after that.

BUG=chromium:1015257
TEST=tast run $(DUT) video.EncodeAccelPerfH264360PI420 on elm

Change-Id: Iaa80779982c0c234ed9100095c9bb4b5b954288a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871150Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707715}

media/gpu/v4l2/v4l2_video_encode_accelerator.cc

@@ -1005,7 +1005,7 @@ void V4L2VideoEncodeAccelerator::PumpBitstreamBuffers() {
       if (bitstream_buffer_pool_.empty()) {
         DVLOGF(4) << "No free bitstream buffer, skip.";
         output_buffer_queue_.push_front(std::move(output_buf));
-        return;
+        break;
       }
 
       auto buffer_ref = std::move(bitstream_buffer_pool_.back());
@@ -1044,6 +1044,13 @@ void V4L2VideoEncodeAccelerator::PumpBitstreamBuffers() {
       IOCTL_OR_ERROR_RETURN(VIDIOC_ENCODER_CMD, &cmd);
     }
   }
+
+  // We may free some V4L2 output buffers above. Enqueue them if needed.
+  if (output_queue_->FreeBuffersCount() > 0) {
+    encoder_thread_.task_runner()->PostTask(
+        FROM_HERE, base::BindOnce(&V4L2VideoEncodeAccelerator::Enqueue,
+                                  base::Unretained(this)));
+  }
 }
 
 bool V4L2VideoEncodeAccelerator::EnqueueInputRecord() {