[Sync] Add HasChildren() function and use it instead of GetFirstChildId()

chrome/browser/sync/glue/session_model_associator.cc

Index: chrome/browser/sync/glue/session_model_associator.cc
diff --git a/chrome/browser/sync/glue/session_model_associator.cc b/chrome/browser/sync/glue/session_model_associator.cc
index edec88d96ce18175da47ad5f1662c5acd08b7945..a6558919591f4d9a98a57a364318b318bee91b95 100644
--- a/chrome/browser/sync/glue/session_model_associator.cc
+++ b/chrome/browser/sync/glue/session_model_associator.cc
@@ -101,7 +101,7 @@ bool SessionModelAssociator::SyncModelHasUserCreatedNodes(bool* has_nodes) {
   }
   // The sync model has user created nodes iff the sessions folder has
   // any children.
-  *has_nodes = root.GetFirstChildId() != sync_api::kInvalidId;
+  *has_nodes = root.HasChildren();
   return true;
 }