Add sequence number to undo steps

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

Index: third_party/WebKit/Source/core/editing/commands/UndoStep.h
diff --git a/third_party/WebKit/Source/core/editing/commands/UndoStep.h b/third_party/WebKit/Source/core/editing/commands/UndoStep.h
index c0e1935eb3f2bc0a27e8b52c84dbd9b2c710ecde..7bf4a503b3f3a03e99cf5c28e4ec4450833eb20a 100644
--- a/third_party/WebKit/Source/core/editing/commands/UndoStep.h
+++ b/third_party/WebKit/Source/core/editing/commands/UndoStep.h
@@ -65,6 +65,8 @@ class UndoStep : public GarbageCollectedFinalized<UndoStep> {
     return m_endingRootEditableElement.get();
   }
 
+  uint64_t sequenceNumber() const { return m_sequenceNumber; }
+
   DECLARE_TRACE();
 
  private:
@@ -80,6 +82,7 @@ class UndoStep : public GarbageCollectedFinalized<UndoStep> {
   Member<Element> m_startingRootEditableElement;
   Member<Element> m_endingRootEditableElement;
   InputEvent::InputType m_inputType;
+  const uint64_t m_sequenceNumber;
 };
 
 }  // namespace blink