The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d36c54bb660c9bfa374a05a222959da11dd1fae4

commit d36c54bb660c9bfa374a05a222959da11dd1fae4
Author: lgcheng <lgcheng@google.com>
Date: Thu Oct 05 19:17:50 2017

Enable Arc Pacakge sync datatype at proper time.

We enable Arc package sync datatype when user enables Arc, and call
StartModels when Arc app instance is ready and Arc packages get
refreshed. But There is a big time gap since we need perform provision
for Arc during opt-in. And this may lead to the high config time-out
rate for Arc packages.

Enable the datatype when provision is successful and reduce the gap
between enabling the sync data type and calling startModels.

Bug:  771838 
Test: Manual test and Arc pacakge related integration test.
Change-Id: I4c70c9d7e3c690fc953dac981398424d166ce51f
Reviewed-on: https://chromium-review.googlesource.com/702014
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Long Cheng <lgcheng@google.com>
Cr-Commit-Position: refs/heads/master@{#506817}
[modify] https://crrev.com/d36c54bb660c9bfa374a05a222959da11dd1fae4/chrome/browser/ui/app_list/arc/arc_package_sync_data_type_controller.cc
[modify] https://crrev.com/d36c54bb660c9bfa374a05a222959da11dd1fae4/chrome/browser/ui/app_list/arc/arc_package_sync_data_type_controller.h

lgcheng@: Is this issue fixed?

Hear no report about sync failure. Mark as fixed for now.

Failures are still recorded in UMA but the datatype should be restarted when Arc becomes ready.