TaskScheduler migration for ServiceWorkerStorage (database task runner)

GetBlockingPool() -> CreateSequencedTaskRunnerWithTraits()

This is BLOCK_SHUTDOWN for now until task runner gains the feature to
mix task shutdown behaviors.

Bug: 667892
Change-Id: Ic15f2bb2730d8b44dac2ade64f062fdca40d517e
Reviewed-on: https://chromium-review.googlesource.com/588607
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489902}
11 files changed