Add sequence number to undo steps

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

 // Copyright 2017 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 "core/editing/commands/UndoStep.h"
 
 #include "core/editing/Editor.h"
 #include "core/editing/commands/EditCommand.h"
 
 namespace blink {
 
+namespace {
+uint64_t s_currentSequenceNumber = 0;
+}
+
 UndoStep* UndoStep::create(Document* document,
                            const VisibleSelection& startingSelection,
                            const VisibleSelection& endingSelection,
                            InputEvent::InputType inputType) {
   return new UndoStep(document, startingSelection, endingSelection, inputType);
 }
 
 UndoStep::UndoStep(Document* document,
                    const VisibleSelection& startingSelection,
                    const VisibleSelection& endingSelection,
                    InputEvent::InputType inputType)
     : m_document(document),
       m_startingSelection(startingSelection),
       m_endingSelection(endingSelection),
       m_startingRootEditableElement(startingSelection.rootEditableElement()),
       m_endingRootEditableElement(endingSelection.rootEditableElement()),
-      m_inputType(inputType) {}
+      m_inputType(inputType),
+      m_sequenceNumber(++s_currentSequenceNumber) {}
 
 void UndoStep::unapply() {
   DCHECK(m_document);
   LocalFrame* frame = m_document->frame();
   DCHECK(frame);
 
   // Changes to the document may have been made since the last editing operation
   // that require a layout, as in <rdar://problem/5658603>. Low level
   // operations, like RemoveNodeCommand, don't require a layout because the high
   // level operations that use them perform one if one is necessary (like for
@@ skipping 54 matching lines @@
 DEFINE_TRACE(UndoStep) {
   visitor->trace(m_document);
   visitor->trace(m_startingSelection);
   visitor->trace(m_endingSelection);
   visitor->trace(m_commands);
   visitor->trace(m_startingRootEditableElement);
   visitor->trace(m_endingRootEditableElement);
 }
 
 }  // namespace blink