[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}
5 files changed