Prefix string16 with base:: in ui/.

ui/views/controls/menu/menu_model_adapter_unittest.cc

Index: ui/views/controls/menu/menu_model_adapter_unittest.cc
diff --git a/ui/views/controls/menu/menu_model_adapter_unittest.cc b/ui/views/controls/menu/menu_model_adapter_unittest.cc
index 6ce3a3762701fb26b6356fa38795aba520af371c..6d2428a92adf5edecb04a65de392d3d47e9c70fc 100644
--- a/ui/views/controls/menu/menu_model_adapter_unittest.cc
+++ b/ui/views/controls/menu/menu_model_adapter_unittest.cc
@@ -51,7 +51,7 @@ class MenuModelBase : public ui::MenuModel {
     return index + command_id_base_;
   }
 
-  virtual string16 GetLabelAt(int index) const OVERRIDE {
+  virtual base::string16 GetLabelAt(int index) const OVERRIDE {
     return items_[index].label;
   }
 
@@ -133,7 +133,7 @@ class MenuModelBase : public ui::MenuModel {
     }
 
     ItemType type;
-    string16 label;
+    base::string16 label;
     ui::MenuModel* submenu;
   };