media/gpu/v4l2vea: delete V4L2 device in device thread

All operations on the V4L2 device should happen in the device thread.
Make sure our reference to the V4L2 device is discarded there so its
destructor can be called in the proper thread.

Bug: None
Test: VEA unittest builds and runs on Kukui in debug mode.

Change-Id: I763389e7bd2946b6ff89f8dffd1e13d7fdd8211c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939995Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719885}

media/gpu/v4l2/v4l2_video_encode_accelerator.cc

@@ -805,6 +805,7 @@ void V4L2VideoEncodeAccelerator::DestroyTask() {
     input_queue_ = nullptr;
     output_queue_ = nullptr;
     image_processor_ = nullptr;
+    device_ = nullptr;
   }
 }