views::Textfield: Implement transpose editing command.

ui/views/controls/textfield/textfield_model.h

Index: ui/views/controls/textfield/textfield_model.h
diff --git a/ui/views/controls/textfield/textfield_model.h b/ui/views/controls/textfield/textfield_model.h
index 135524e289a44035be219830cdb0e1044f7febf1..df3e9754e2d52c1e51949b542cf3d2552c56dfc4 100644
--- a/ui/views/controls/textfield/textfield_model.h
+++ b/ui/views/controls/textfield/textfield_model.h
@@ -177,6 +177,11 @@ class VIEWS_EXPORT TextfieldModel {
   // if any text is pasted.
   bool Paste();
 
+  // Transposes the characters to either side of the insertion point and
+  // advances the insertion point past both of them. Returns true if text is
+  // changed.
+  bool Transpose();
+
   // Tells if any text is selected, even if the selection is in composition
   // text.
   bool HasSelection() const;