[TaskScheduler] Make PackExtensionJob run on sequences.

It used to run on FILE thread, which is deprecated.

This CL also removes ref-counting from PackExtensionJob, as
PackExtensionJob::Client should be responsible for keeping
PackExtensionJob alive.

Change set_asynchronous(bool) -> set_synchronous as async is
the default, and set_synchronous was always called with false.

Bug: 689520
Change-Id: I87143df26e7adf28bbbe6fd379a38d17d07cb967
Reviewed-on: https://chromium-review.googlesource.com/611044
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493900}
7 files changed