media/gpu/VaapiVD: Don't set decoder thread's task runner on re-initialization.

This CL makes a small change to the VaapiVideoDecoder to not set the decoder
thread's task runner and call set_parent_task_runner() on the video frame pool
upon re-initialization. They are redundant and cause an assert in the video
frame pool when executing in debug mode.

TEST=./video_decode_accelerator_tests --use_vd on eve

BUG=976507

Change-Id: Idb3712e5d835c4aa122ce7bc9f7efd2dd99600a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1686666
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683913}

media/gpu/vaapi/vaapi_video_decoder.cc

@@ -124,7 +124,6 @@ int VaapiVideoDecoder::GetMaxDecodeRequests() const {
   return kMaxDecodeRequests;
 }
 
-// TODO(dstaessens): Handle re-initialization.
 void VaapiVideoDecoder::Initialize(const VideoDecoderConfig& config,
                                    bool low_delay,
                                    CdmContext* cdm_context,
@@ -155,8 +154,10 @@ void VaapiVideoDecoder::Initialize(const VideoDecoderConfig& config,
     return;
   }
 
-  decoder_thread_task_runner_ = decoder_thread_.task_runner();
-  frame_pool_->set_parent_task_runner(decoder_thread_task_runner_);
+  if (!decoder_thread_task_runner_) {
+    decoder_thread_task_runner_ = decoder_thread_.task_runner();
+    frame_pool_->set_parent_task_runner(decoder_thread_task_runner_);
+  }
 
   decoder_thread_task_runner_->PostTask(
       FROM_HERE,