Add GetDefaultIndex() to ComboboxModel.

ui/base/models/combobox_model.h

Index: ui/base/models/combobox_model.h
diff --git a/ui/base/models/combobox_model.h b/ui/base/models/combobox_model.h
index a746e5311a0c8e75670760da174e5dfe41cff254..8573740e640ea29e1a683ac55f6757f64482d508 100644
--- a/ui/base/models/combobox_model.h
+++ b/ui/base/models/combobox_model.h
@@ -6,6 +6,7 @@
 #define UI_BASE_MODELS_COMBOBOX_MODEL_H_
 
 #include "base/string16.h"
+#include "ui/base/ui_export.h"
 
 namespace ui {
 
@@ -18,6 +19,10 @@ class ComboboxModel {
   // Returns the string at the specified index.
   virtual string16 GetItemAt(int index) = 0;
 
+  // The index of the item that is selected by default (before user
+  // interaction).
+  virtual int GetDefaultIndex() const;
+
  protected:
   virtual ~ComboboxModel() {}
 };