Do not sync the ARC++ opt-in preference.

This is a re-implementation of 1c7841e8e0322b8c3599710e017ff0fb38ab056f,
which made ARC opt-in preference not synced.  That change also stopped
sending UMA stats due to not calling the OnSyncedPrefChanged method.

BUG=633017
TEST=browser_tests --gtest_filter=ArcAuthServiceTest.*
Additionally manually tested that the normal opt-in/opt-out process
worked as expected.
NOTRY=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2206883002
Cr-Commit-Position: refs/heads/master@{#410994}
(cherry picked from commit 176f6733ab5bdf6de91f08e840b814cf76b910bb)

Review-Url: https://codereview.chromium.org/2235313003
Cr-Commit-Position: refs/branch-heads/2785@{#583}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}
1 file changed