Root cause, there's no change event for synced printers so they're not propagated to the cache.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/330f5e14d2b1e3ac27cd3f11272f20e13b013474

commit 330f5e14d2b1e3ac27cd3f11272f20e13b013474
Author: Sean Kau <skau@chromium.org>
Date: Tue Aug 22 17:28:41 2017

Prepopulate CupsPrintersManager cache with Synced printers.

SyncedPrintersManager only emits OnPrintersChanged if printers actually
change.  Populate the printer cache in CupsPrintersManager when we
create it.

Bug:  757672 
Change-Id: I83438dd6a6f7d2d3925095c671427aa8881b7688
Reviewed-on: https://chromium-review.googlesource.com/625277
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496347}
[modify] https://crrev.com/330f5e14d2b1e3ac27cd3f11272f20e13b013474/chrome/browser/chromeos/printing/cups_printers_manager.cc

This bug requires manual review: We are only 13 days from stable.
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), ketakid@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approving merge to M61 Chrome OS.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d81764377f8e6996de5ffe96b0861c27878e3c38

commit d81764377f8e6996de5ffe96b0861c27878e3c38
Author: Sean Kau <skau@chromium.org>
Date: Tue Aug 22 19:55:36 2017

Prepopulate CupsPrintersManager cache with Synced printers.

SyncedPrintersManager only emits OnPrintersChanged if printers actually
change.  Populate the printer cache in CupsPrintersManager when we
create it.

TBR=skau@chromium.org

(cherry picked from commit 330f5e14d2b1e3ac27cd3f11272f20e13b013474)

Bug:  757672 
Change-Id: I83438dd6a6f7d2d3925095c671427aa8881b7688
Reviewed-on: https://chromium-review.googlesource.com/625277
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#496347}
Reviewed-on: https://chromium-review.googlesource.com/627105
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#772}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[modify] https://crrev.com/d81764377f8e6996de5ffe96b0861c27878e3c38/chrome/browser/chromeos/printing/cups_printers_manager.cc