[TaskScheduler] Make ExternalPrefLoader use sequence instead of FILE thread.

Bug: 689520
Change-Id: I65adecbed9babdb9d09fdf05f3daebcb966360ac
Reviewed-on: https://chromium-review.googlesource.com/587669
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491584}
1 file changed