[Sync] Add context proto and directory support

sync/syncable/directory_backing_store.h

Index: sync/syncable/directory_backing_store.h
diff --git a/sync/syncable/directory_backing_store.h b/sync/syncable/directory_backing_store.h
index 5b96848e3256456ba09ea0e40487b761cf95696a..dfacebec8fd2f5950a90aa99f18b35fde9dd05e2 100644
--- a/sync/syncable/directory_backing_store.h
+++ b/sync/syncable/directory_backing_store.h
@@ -88,6 +88,7 @@ class SYNC_EXPORT_PRIVATE DirectoryBackingStore : public base::NonThreadSafe {
   bool CreateModelsTable();
   bool CreateV71ModelsTable();
   bool CreateV75ModelsTable();
+  bool CreateV81ModelsTable();
 
   // We don't need to load any synced and applied deleted entries, we can
   // in fact just purge them forever on startup.
@@ -171,6 +172,7 @@ class SYNC_EXPORT_PRIVATE DirectoryBackingStore : public base::NonThreadSafe {
   bool MigrateVersion84To85();
   bool MigrateVersion85To86();
   bool MigrateVersion86To87();
+  bool MigrateVersion87To88();
 
   scoped_ptr<sql::Connection> db_;
   sql::Statement save_meta_statment_;