Remove most occurences of PrefRegistrySyncable::UNSYNCABLE_PREF

Remove most occurences of PrefRegistrySyncable::UNSYNCABLE_PREF

Now that we have added PrefRegistry registration functions that do not need to
accept a flags argument, passing PrefRegistrySyncable::UNSYNCABLE_PREF is
redundant and we can remove it altogether.

Ran the following command:
for x in $(git ls-files | grep -E "\.(cc|h|mm)$") ; do perl -0777 -i -pe 's/,[ \n]*(user_prefs::)+PrefRegistrySyncable::UNSYNCABLE_PREF//igs' $x ; done && git cl format

There are still some occurences that need to be manually fixed.

BUG=476800
Committed: https://crrev.com/aa60872e346eaf354bb15dbd698546b1bdddd132
Cr-Commit-Position: refs/heads/master@{#326993}

[raymes, 2015-04-23 07:41:42]

raymes@chromium.org changed reviewers:
+ mnissler@chromium.org

[Mattias Nissler (ping if slow), 2015-04-23 07:54:12]

Sweet! LGTM

[raymes, 2015-04-24 00:53:40]

raymes@chromium.org changed reviewers:
+ brettw@chromium.org

[raymes, 2015-04-24 00:53:41]

+brettw for top level OWNERS

[brettw, 2015-04-24 17:25:23]

LGTM rubberstamp

[raymes, 2015-04-27 01:45:30]

The CQ bit was checked by raymes@chromium.org

[raymes, 2015-04-27 01:45:30]

The patchset sent to the CQ was uploaded after l-g-t-m from brettw@chromium.org,
mnissler@chromium.org
Link to the patchset: https://codereview.chromium.org/1102733002/#ps20001
(title: " ")

[commit-bot: I haz the power, 2015-04-27 01:45:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1102733002/20001

[commit-bot: I haz the power, 2015-04-27 03:00:37]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-27 03:01:37]

Patchset 2 (id:??) landed as
https://crrev.com/aa60872e346eaf354bb15dbd698546b1bdddd132
Cr-Commit-Position: refs/heads/master@{#326993}