Mark CONTENT_SETTINGS_TYPE_PUSH_MESSAGING as UNSYNCABLE

The privacy considerations around push messaging are device-specific, so
the permission should not be synced. Also, this aligns push messaging
with CONTENT_SETTINGS_TYPE_NOTIFICATIONS, which is already UNSYNCABLE.

BUG=567153

Review URL: https://codereview.chromium.org/1506613003

Cr-Commit-Position: refs/heads/master@{#363520}
1 file changed