Add sequence number to undo steps

third_party/WebKit/Source/core/editing/commands/UndoStack.cpp

 /*
  * Copyright (C) 2006, 2007 Apple, Inc.  All rights reserved.
  * Copyright (C) 2012 Google, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 27 matching lines @@
 // into a single action.
 static const size_t maximumUndoStackDepth = 1000;
 
 UndoStack::UndoStack() : m_inRedo(false) {}
 
 UndoStack* UndoStack::create() {
   return new UndoStack();
 }
 
 void UndoStack::registerUndoStep(UndoStep* step) {
+  if (m_undoStack.size())
+    DCHECK_GE(step->sequenceNumber(), m_undoStack.back()->sequenceNumber());
   if (m_undoStack.size() == maximumUndoStackDepth)
     m_undoStack.removeFirst();  // drop oldest item off the far end
   if (!m_inRedo)
     m_redoStack.clear();
   m_undoStack.append(step);
 }
 
 void UndoStack::registerRedoStep(UndoStep* step) {
   m_redoStack.append(step);
 }
@@ skipping 33 matching lines @@
   m_undoStack.clear();
   m_redoStack.clear();
 }
 
 DEFINE_TRACE(UndoStack) {
   visitor->trace(m_undoStack);
   visitor->trace(m_redoStack);
 }
 
 }  // namespace blink