Remove dependency of PrefSyncableService on Profile.
In preparation of componentization of PrefSyncableService (with the goal
of sharing the code with iOS embedder), remove the dependency on Profile
by converting the two helper static method to free functions and moving
them to chrome/browser/prefs/pref_service_syncable_util.{cc,h} that will
stay in chrome/browser/prefs.
BUG=522536
TBR=thakis@chromium.org
Review URL: https://codereview.chromium.org/
1326353002
Cr-Commit-Position: refs/heads/master@{#348600}