Add a mojo bridge for PersistentPrefStore.

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..04221aaecec895bb31370c3b484a947328eedf77 100644
--- a/mojo/public/cpp/bindings/sync_call_restrictions.h
+++ b/mojo/public/cpp/bindings/sync_call_restrictions.h
@@ -15,6 +15,10 @@
 #define ENABLE_SYNC_CALL_RESTRICTIONS 0
 #endif
 
+namespace content {
+class BrowserMainRunnerImpl;
+}
+
 namespace leveldb {
 class LevelDBMojoProxy;
 }
@@ -58,6 +62,8 @@ 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;
+  // Sync calls are enabled before the main MessageLoop starts.
+  friend class content::BrowserMainRunnerImpl;
   // END ALLOWED USAGE.
 
   // BEGIN USAGE THAT NEEDS TO BE FIXED.