Add sequence number to undo steps

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

Index: third_party/WebKit/Source/core/editing/commands/UndoStack.h
diff --git a/third_party/WebKit/Source/core/editing/commands/UndoStack.h b/third_party/WebKit/Source/core/editing/commands/UndoStack.h
index 8f7e3c837d17fa4cdb9c5a298650be0ea64925d0..3b69a73b341f14bd2b2cd1b3575cf690f101e955 100644
--- a/third_party/WebKit/Source/core/editing/commands/UndoStack.h
+++ b/third_party/WebKit/Source/core/editing/commands/UndoStack.h
@@ -55,6 +55,7 @@ class UndoStack final : public GarbageCollected<UndoStack> {
   void undo();
   void redo();
   void clear();
+  uint64_t maxSequence() const { return m_maxSequence; }
 
   DECLARE_TRACE();
 
@@ -63,6 +64,7 @@ class UndoStack final : public GarbageCollected<UndoStack> {
 
   typedef HeapDeque<Member<UndoStep>> UndoStepStack;
 
+  uint64_t m_maxSequence;
   bool m_inRedo;
   UndoStepStack m_undoStack;
   UndoStepStack m_redoStack;