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

sync/syncable/parent_child_index_unittest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sync/syncable/parent_child_index.h"
 
 #include <list>
 
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 371 matching lines @@
   EntryKernel* p1 = MakeUniqueClientItem(PREFERENCES, 1);
   EntryKernel* p2 = MakeUniqueClientItem(PREFERENCES, 2);
 
   index_.Insert(p1);
   index_.Insert(p2);
 
   EXPECT_TRUE(index_.Contains(p1));
   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));
   EXPECT_TRUE(index_.Contains(p2));
   children = index_.GetChildren(type_root_id);
   ASSERT_TRUE(children);
   EXPECT_EQ(1UL, children->size());
 
   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);
   EXPECT_EQ(Id(), IndexKnownModelTypeRootId(PREFERENCES));
 }
 
 }  // namespace syncable
 }  // namespace syncer