Add sequence number to undo steps

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

Index: third_party/WebKit/Source/core/editing/commands/UndoStack.cpp
diff --git a/third_party/WebKit/Source/core/editing/commands/UndoStack.cpp b/third_party/WebKit/Source/core/editing/commands/UndoStack.cpp
index 7f435982f87767a5c40c03c6590b17436a7cefc5..e492b81869c1569e2f3a0c67e283336a46c2a1ea 100644
--- a/third_party/WebKit/Source/core/editing/commands/UndoStack.cpp
+++ b/third_party/WebKit/Source/core/editing/commands/UndoStack.cpp
@@ -38,7 +38,7 @@ namespace blink {
 // into a single action.
 static const size_t maximumUndoStackDepth = 1000;
 
-UndoStack::UndoStack() : m_inRedo(false) {}
+UndoStack::UndoStack() : m_maxSequence(0), m_inRedo(false) {}
 
 UndoStack* UndoStack::create() {
   return new UndoStack();
@@ -47,8 +47,10 @@ UndoStack* UndoStack::create() {
 void UndoStack::registerUndoStep(UndoStep* step) {
   if (m_undoStack.size() == maximumUndoStackDepth)
     m_undoStack.removeFirst();  // drop oldest item off the far end
-  if (!m_inRedo)
+  if (!m_inRedo) {
     m_redoStack.clear();
+    step->setSequence(++m_maxSequence);

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

Usage of |m_maxSequence| isn't clear. Why increment here?

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

This is the place where a new undo step is pushed to the stack.
We don't want to set the sequence of an undo step if it's just popped from the
redo stack.

+  }
   m_undoStack.append(step);
 }