Remove usage of SequencedWorkerPool::GetNamedSequenceToken from WallpaperApi and WallpaperPrivateApi

SequencedWorkerPool is being deprecated in favor of TaskScheduler.

Bug: 667892
Change-Id: Id4b1bdfba4b5f004bea8985925f23d5211016280
Reviewed-on: https://chromium-review.googlesource.com/561657
Commit-Queue: Biao She <bshe@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490672}
7 files changed