Remove deprecated priority-less SequencedWorkerPool constructor.

And migrate one callsite missed in https://codereview.chromium.org/2077413009:
 - arc_process_service.cc

After last two callers are removed:
 - https://codereview.chromium.org/2103883004
 - https://codereview.chromium.org/2197753002/

BUG=622400
TBR=danakj@chromium.org

Review-Url: https://codereview.chromium.org/2199353002
Cr-Commit-Position: refs/heads/master@{#411905}
3 files changed