Make the ARC opt-in preference not synced.

Make the ARC opt-in preference not synced.

If we sync the ARC opt-in prefence we may cause unintentional data loss
when the user opts-out of ARC on one device.

BUG=633017
TEST=Manually test by opting out of ARC on one machine and observing a
second (opted-in) machine's state.

Committed: https://crrev.com/1c7841e8e0322b8c3599710e017ff0fb38ab056f
Cr-Commit-Position: refs/heads/master@{#408909}

[dspaid, 2016-08-01 01:53:37]

dspaid@chromium.org changed reviewers:
+ hidehiko@chromium.org

[hidehiko, 2016-08-01 03:50:20]

lgtm

[dspaid, 2016-08-01 03:54:40]

The CQ bit was checked by dspaid@chromium.org

[commit-bot: I haz the power, 2016-08-01 03:54:47]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-01 04:42:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-08-01 04:44:28]

Description was changed from

==========
Make the ARC opt-in preference not synced.

If we sync the ARC opt-in prefence we may cause unintentional data loss
when the user opts-out of ARC on one device.

BUG=633017
TEST=Manually test by opting out of ARC on one machine and observing a
second (opted-in) machine's state.
==========

to

==========
Make the ARC opt-in preference not synced.

If we sync the ARC opt-in prefence we may cause unintentional data loss
when the user opts-out of ARC on one device.

BUG=633017
TEST=Manually test by opting out of ARC on one machine and observing a
second (opted-in) machine's state.

Committed: https://crrev.com/1c7841e8e0322b8c3599710e017ff0fb38ab056f
Cr-Commit-Position: refs/heads/master@{#408909}
==========

[commit-bot: I haz the power, 2016-08-01 04:44:29]

Patchset 1 (id:??) landed as
https://crrev.com/1c7841e8e0322b8c3599710e017ff0fb38ab056f
Cr-Commit-Position: refs/heads/master@{#408909}

[khmel, 2016-08-04 18:29:30]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/2215943002/ by khmel@chromium.org.

The reason for reverting is: This introduces UI regression (Arc start
notification is shown on each device on each update). Also this breaks UMA
statistics..