vea: Fix flush function of V4L2 VEA.

Originally, VEA calls the flush callback before notifying the client
that the last frame buffer is ready. This CL changed the calling
order to ensure all frames buffer are sent back to the client before
VEA notifies the flush is completed.

BUG=b:71882314
TEST=run video_encode_accelerator_unittest at device

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I235411734dff3200df3b36b1f14211fbd0d41c8f
Reviewed-on: https://chromium-review.googlesource.com/956653Reviewed-by: Pawel Osciak <posciak@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542059}

media/gpu/v4l2/v4l2_video_encode_accelerator.cc

@@ -743,17 +743,6 @@ void V4L2VideoEncodeAccelerator::Dequeue() {
       NOTIFY_ERROR(kPlatformFailureError);
       return;
     }
-    if ((encoder_state_ == kFlushing) && (dqbuf.flags & V4L2_BUF_FLAG_LAST)) {
-      DVLOGF(3) << "Flush completed. Start the encoder again.";
-      encoder_state_ = kEncoding;
-      // Notify client that flush has finished successfully.
-      std::move(flush_callback_).Run(true);
-      // Start the encoder again.
-      struct v4l2_encoder_cmd cmd;
-      memset(&cmd, 0, sizeof(cmd));
-      cmd.cmd = V4L2_ENC_CMD_START;
-      IOCTL_OR_ERROR_RETURN(VIDIOC_ENCODER_CMD, &cmd);
-    }
     const bool key_frame = ((dqbuf.flags & V4L2_BUF_FLAG_KEYFRAME) != 0);
     OutputRecord& output_record = output_buffer_map_[dqbuf.index];
     DCHECK(output_record.at_device);
@@ -776,6 +765,19 @@ void V4L2VideoEncodeAccelerator::Dequeue() {
                                   dqbuf.timestamp.tv_usec +
                                   dqbuf.timestamp.tv_sec *
                                       base::Time::kMicrosecondsPerSecond)));
+    if ((encoder_state_ == kFlushing) && (dqbuf.flags & V4L2_BUF_FLAG_LAST)) {
+      // Notify client that flush has finished successfully. The flush callback
+      // should be called after notifying the last buffer is ready.
+      DVLOGF(3) << "Flush completed. Start the encoder again.";
+      encoder_state_ = kEncoding;
+      child_task_runner_->PostTask(
+          FROM_HERE, base::BindOnce(std::move(flush_callback_), true));
+      // Start the encoder again.
+      struct v4l2_encoder_cmd cmd;
+      memset(&cmd, 0, sizeof(cmd));
+      cmd.cmd = V4L2_ENC_CMD_START;
+      IOCTL_OR_ERROR_RETURN(VIDIOC_ENCODER_CMD, &cmd);
+    }
 
     output_record.at_device = false;
     free_output_buffers_.push_back(dqbuf.index);