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 4aa768978e4bf149ce539dc85ebfd7fdee6d256e..73c27813d1f0635372d6ef1963d49b887b25f41b 100644
--- a/third_party/WebKit/Source/core/editing/commands/UndoStep.h
+++ b/third_party/WebKit/Source/core/editing/commands/UndoStep.h
@@ -47,6 +47,8 @@ class UndoStep : public GarbageCollectedFinalized<UndoStep> {
   virtual void unapply() = 0;
   virtual void reapply() = 0;
   virtual InputEvent::InputType inputType() const = 0;
+  virtual int sequence() const = 0;

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

Do we really want to keep |UndoStep| as interface?

I think it is better to have sequence number as const member variable
initialized in ctor.


BTW,
We should get rid of GarbageCollectedFinalized<UndoStep> since it doesn't have
member variables to track.

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

Do you mean removing |UndoStep| and using |EditCommandComposition| directly? In
fact what's the reason of having |UndoStep|?
Is the construction order of the steps same as the order they are pushed to undo
stack? If so, then yes.

+  virtual void setSequence(int) = 0;
 };
 
 }  // namespace blink