Remove usage of SequencedWorkerPool in GN.

SequencedWorkerPool is deprecated. A process-wide TaskScheduler
must be used instead.

Bug: 667892
Change-Id: I67f5bafe5e5eaba3aeb40664dea7cdd7d4250b60
Reviewed-on: https://chromium-review.googlesource.com/879354
Commit-Queue: Fran├žois Doray <fdoray@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531270}
22 files changed