GitLab

Change ExternalPrinters to perform slow operations off the UI thread
to avoid introducing jank.  Also add infrastructure to observe
preferences and manage instances by user.

Bug: 758680
Change-Id: I56f4dae9f0c2062cfe177fcdac88785978874217
Reviewed-on: https://chromium-review.googlesource.com/826705
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Xiaoqian Dai OOO until Jan 2nd <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526087}