[Sync] Make GetFirstChildId return a flag indicating success

chrome/browser/sync/internal_api/change_reorder_buffer.h

 // Copyright (c) 2011 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.
 //
 // Defines ChangeReorderBuffer, which can be used to sort a list of item
 // actions to achieve the ordering constraint required by the SyncObserver
 // interface of the SyncAPI.
 
 #ifndef CHROME_BROWSER_SYNC_INTERNAL_API_CHANGE_REORDER_BUFFER_H_
 #define CHROME_BROWSER_SYNC_INTERNAL_API_CHANGE_REORDER_BUFFER_H_
 #pragma once
 
 #include <map>
 #include <vector>
 
+#include "base/compiler_specific.h"
 #include "base/memory/linked_ptr.h"
 #include "chrome/browser/sync/internal_api/base_transaction.h"
 #include "chrome/browser/sync/internal_api/change_record.h"
 #include "chrome/browser/sync/protocol/sync.pb.h"
 
 namespace sync_api {
 
 // ChangeReorderBuffer is a utility type which accepts an unordered set
 // of changes (via its Push methods), and yields an ImmutableChangeRecordList
 // (via the GetAllChangesInTreeOrder method) that are in the order that
@@ skipping 51 matching lines @@
   // Reset the buffer, forgetting any pushed items, so that it can be used
   // again to reorder a new set of changes.
   void Clear() {
     operations_.clear();
   }
 
   bool IsEmpty() const {
     return operations_.empty();
   }
 
-  // Output a reordered list of changes to |changelist| using the items that
-  // were pushed into the reorder buffer. |sync_trans| is used to determine the
-  // ordering.
-  ImmutableChangeRecordList
-      GetAllChangesInTreeOrder(const BaseTransaction* sync_trans);
+  // Output a reordered list of changes to |changes| using the items
+  // that were pushed into the reorder buffer. |sync_trans| is used to
+  // determine the ordering.  Returns true if successful, or false if
+  // an error was encountered.
+  bool GetAllChangesInTreeOrder(
+      const BaseTransaction* sync_trans,
+      ImmutableChangeRecordList* changes) WARN_UNUSED_RESULT;
 
  private:
   class Traversal;
   enum Operation {
     OP_ADD,                             // AddedItem.
     OP_DELETE,                          // DeletedItem.
     OP_UPDATE_PROPERTIES_ONLY,          // UpdatedItem with position_changed=0.
     OP_UPDATE_POSITION_AND_PROPERTIES,  // UpdatedItem with position_changed=1.
   };
   typedef std::map<int64, Operation> OperationMap;
@@ skipping 10 matching lines @@
 
   // Stores type-specific extra data per-ID.
   ExtraDataMap extra_data_;
 
   DISALLOW_COPY_AND_ASSIGN(ChangeReorderBuffer);
 };
 
 }  // namespace sync_api
 
 #endif  // CHROME_BROWSER_SYNC_INTERNAL_API_CHANGE_REORDER_BUFFER_H_