Add sequence number to undo steps

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

Index: third_party/WebKit/Source/core/editing/commands/CompositeEditCommand.h
diff --git a/third_party/WebKit/Source/core/editing/commands/CompositeEditCommand.h b/third_party/WebKit/Source/core/editing/commands/CompositeEditCommand.h
index 24d67a7746c024858645e740496c0378033f37c8..7a522abc0ba3aeb1cf6ed46a0ae1135a06316e6f 100644
--- a/third_party/WebKit/Source/core/editing/commands/CompositeEditCommand.h
+++ b/third_party/WebKit/Source/core/editing/commands/CompositeEditCommand.h
@@ -55,6 +55,9 @@ class EditCommandComposition final : public UndoStep {
   void unapply() override;
   void reapply() override;
   InputEvent::InputType inputType() const override;
+  int sequence() const override;
+  void setSequence(int) override;
+
   void append(SimpleEditCommand*);
   void append(EditCommandComposition*);
 
@@ -86,6 +89,7 @@ class EditCommandComposition final : public UndoStep {
   Member<Element> m_startingRootEditableElement;
   Member<Element> m_endingRootEditableElement;
   InputEvent::InputType m_inputType;
+  int m_sequence;
 };
 
 class CORE_EXPORT CompositeEditCommand : public EditCommand {