Revert "[TaskScheduler] Migrate some extension installer code to TaskScheduler."

This reverts commit 05dabf99bbe01112e7a78716869d762aa614ff9a.

Reason for revert: Data race https://crbug.com/750382 for details

Original change's description:
> [TaskScheduler] Migrate some extension installer code to TaskScheduler.
> 
> Bug: 689520
> Change-Id: I8743112968e261459d8aee5491e2280da1461661
> Reviewed-on: https://chromium-review.googlesource.com/585386
> Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
> Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#489747}

TBR=lazyboy@chromium.org,rdevlin.cronin@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

NOPRESUBMIT=True

Bug: 689520, 750382 
Change-Id: Ic7065c7ebbae93997f5008164d8c555d032d3150
Reviewed-on: https://chromium-review.googlesource.com/591935
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490607}
5 files changed