Pref service: enable for user prefs in chrome behind a flag.

mojo/public/cpp/bindings/sync_call_restrictions.h

Index: mojo/public/cpp/bindings/sync_call_restrictions.h
diff --git a/mojo/public/cpp/bindings/sync_call_restrictions.h b/mojo/public/cpp/bindings/sync_call_restrictions.h
index 39a77a8518bd4b391422997b80ca3d4a95c998e4..55290427845ab7f2817189fe42c9ab4dab1651c6 100644
--- a/mojo/public/cpp/bindings/sync_call_restrictions.h
+++ b/mojo/public/cpp/bindings/sync_call_restrictions.h
@@ -19,6 +19,10 @@ namespace leveldb {
 class LevelDBMojoProxy;
 }
 
+namespace prefs {
+class PersistentPrefStoreClient;
+}
+
 namespace ui {
 class Gpu;
 }
@@ -58,6 +62,9 @@ class MOJO_CPP_BINDINGS_EXPORT SyncCallRestrictions {
   friend class ui::Gpu;  // http://crbug.com/620058
   // LevelDBMojoProxy makes same-process sync calls from the DB thread.
   friend class leveldb::LevelDBMojoProxy;
+  // Pref service connection is sync at startup.
+  friend class prefs::PersistentPrefStoreClient;
+
   // END ALLOWED USAGE.
 
   // BEGIN USAGE THAT NEEDS TO BE FIXED.