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..21adfad0a6706d65a770249d25a32364f356a845 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();
+  int maxSequence() const { return m_maxSequence; }
 
   DECLARE_TRACE();
 
@@ -63,6 +64,7 @@ class UndoStack final : public GarbageCollected<UndoStack> {
 
   typedef HeapDeque<Member<UndoStep>> UndoStepStack;
 
+  int m_maxSequence;

[yosin_UTC9, 2017/01/17 08:13:29]

Should we use |uint64_t| or |size_t|? But, I'm not sure. Could you explain
rationale to use |int|?

[Xiaocheng, 2017/01/17 09:04:02]

No special rationale, just an arbitrary choice.

Let's use |uint64_t| to be on the safe side.

   bool m_inRedo;
   UndoStepStack m_undoStack;
   UndoStepStack m_redoStack;