Enable Null Syncable ID which is different than Root ID.

sync/syncable/mutable_entry.cc

Index: sync/syncable/mutable_entry.cc
diff --git a/sync/syncable/mutable_entry.cc b/sync/syncable/mutable_entry.cc
index 0aa37e7a4df1369cb2ad2e91cdd3c36fe39eb358..0cc06f90b69ea8524d301c9b8f238f8697534693 100644
--- a/sync/syncable/mutable_entry.cc
+++ b/sync/syncable/mutable_entry.cc
@@ -227,10 +227,14 @@ void MutableEntry::PutUniquePosition(const UniquePosition& value) {
 }
 
 bool MutableEntry::PutPredecessor(const Id& predecessor_id) {
-  MutableEntry predecessor(write_transaction(), GET_BY_ID, predecessor_id);
-  if (!predecessor.good())
-    return false;
-  dir()->PutPredecessor(kernel_, predecessor.kernel_);
+  if (predecessor_id.IsNull()) {
+    dir()->PutPredecessor(kernel_, NULL);
+  } else {
+    MutableEntry predecessor(write_transaction(), GET_BY_ID, predecessor_id);
+    if (!predecessor.good())
+      return false;
+    dir()->PutPredecessor(kernel_, predecessor.kernel_);
+  }
   return true;
 }