Remove reference to deleted sequenced_worker_pool

Bug: 815232
Change-Id: I7fad2ccc0c63b16839fb4f43b095bbfb41e844fd
Reviewed-on: https://chromium-review.googlesource.com/935156
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Tim Brown <timbrown@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#538882}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 99637d39f360b06fa4d3692f837637a2203d4c75