media: Fix GPU crash when reloading video.

Originally we didn't keep the ownership of |gpu_task_runner_| when
creating VideoDecoder. When we create the second VideoDecoder, this
variable will become nullptr. This CL fixes this bug.

BUG=chromium:986752
TEST=reload crosvideo.appspot.com and no crash

Change-Id: I8abebd44b7423dff0bcb663ae74fdfe7e958e7fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714625Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680281}

media/mojo/services/gpu_mojo_media_client.cc

@@ -228,7 +228,7 @@ std::unique_ptr<VideoDecoder> GpuMojoMediaClient::CreateVideoDecoder(
         auto frame_converter = std::make_unique<MailboxVideoFrameConverter>(
             base::BindRepeating(&DmabufVideoFramePool::UnwrapFrame,
                                 base::Unretained(frame_pool.get())),
-            std::move(gpu_task_runner_),
+            gpu_task_runner_,
             base::BindOnce(&GetCommandBufferStub, gpu_task_runner_,
                            media_gpu_channel_manager_,
                            command_buffer_id->channel_token,