Migrate history_report to TaskScheduler API

GetBlockingPool() is replaced by SequencedTaskRunner. There's no
per-task trait in SequencedTaskRunner, so the shutdown behavior are
all set to BLOCK_SHUTDOWN.

Bug: 667892
Change-Id: I6e8f2507b201a8cb077d7338f504684ca1e232c8
Reviewed-on: https://chromium-review.googlesource.com/580620
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488836}
4 files changed