media/gpu/v4l2svd: directly call change resolution callback

The ApplyResolutionChange() method is already called in the right
sequence, so we can call our own callback directly instead of posting it
as a task.

BUG=b:149663704
TEST=VDAtest --use_vd passing on Kukui.

Change-Id: I78407ecfa42c79753263ae09c5542caa525af6fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143424
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759128}

media/gpu/v4l2/v4l2_slice_video_decoder.cc

@@ -430,8 +430,7 @@ void V4L2SliceVideoDecoder::ApplyResolutionChange() {
   DVLOGF(3);
   DCHECK(continue_change_resolution_cb_);
 
-  decoder_task_runner_->PostTask(FROM_HERE,
-                                 std::move(continue_change_resolution_cb_));
+  std::move(continue_change_resolution_cb_).Run();
 }
 
 void V4L2SliceVideoDecoder::ContinueChangeResolution(