Sync: Handle empty (implicit) ParentId in GetPredecessorId and GetSuccessorId

sync/syncable/parent_child_index_unittest.cc

Index: sync/syncable/parent_child_index_unittest.cc
diff --git a/sync/syncable/parent_child_index_unittest.cc b/sync/syncable/parent_child_index_unittest.cc
index 2519aab7d4edb2ed80ecd55063d72b8fdb9ffd90..6362242f2481c9a933c6da01d3817fc7327d9b41 100644
--- a/sync/syncable/parent_child_index_unittest.cc
+++ b/sync/syncable/parent_child_index_unittest.cc
@@ -389,10 +389,13 @@ TEST_F(ParentChildIndexTest, NodesWithImplicitParentId) {
   EXPECT_TRUE(index_.Contains(p2));
 
   // Items should appear under the type root
-  const OrderedChildSet* children = index_.GetChildren(type_root_id);
+  const OrderedChildSet* children = index_.GetChildren(type_root);
   ASSERT_TRUE(children);
   EXPECT_EQ(2UL, children->size());
 
+  EXPECT_EQ(2UL, index_.GetSiblings(p1)->size());
+  EXPECT_EQ(2UL, index_.GetSiblings(p2)->size());
+
   index_.Remove(p1);
 
   EXPECT_FALSE(index_.Contains(p1));
@@ -404,7 +407,7 @@ TEST_F(ParentChildIndexTest, NodesWithImplicitParentId) {
   index_.Remove(p2);
 
   EXPECT_FALSE(index_.Contains(p2));
-  children = index_.GetChildren(type_root_id);
+  children = index_.GetChildren(type_root);
   ASSERT_EQ(nullptr, children);
 
   index_.Remove(type_root);