Increase priority of OffloadingVideoEncoder's default runner

VideoEncoder might be used for live video or something else that a user is waiting for.
This priority is suggested by Gabriel Charette.

Bug: 1132057
Change-Id: I893b6a17df0cfeee368b8ee5a87afe12912a1ed2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508516
Auto-Submit: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822713}

media/base/offloading_video_encoder.cc

@@ -29,7 +29,7 @@ OffloadingVideoEncoder::OffloadingVideoEncoder(
     std::unique_ptr<VideoEncoder> wrapped_encoder)
     : OffloadingVideoEncoder(std::move(wrapped_encoder),
                              base::ThreadPool::CreateSequencedTaskRunner(
-                                 {base::TaskPriority::USER_VISIBLE}),
+                                 {base::TaskPriority::USER_BLOCKING}),
                              base::SequencedTaskRunnerHandle::Get()) {}
 
 void OffloadingVideoEncoder::Initialize(VideoCodecProfile profile,