Add base:: to string16s in win8/.

win8/metro_driver/metro_dialog_box.h

Index: win8/metro_driver/metro_dialog_box.h
diff --git a/win8/metro_driver/metro_dialog_box.h b/win8/metro_driver/metro_dialog_box.h
index 5a6a94dbc633d8f703a593fc4065abe9d6e63845..a8ee5b6604e491f209877d90964f68204236d726 100644
--- a/win8/metro_driver/metro_dialog_box.h
+++ b/win8/metro_driver/metro_dialog_box.h
@@ -16,10 +16,10 @@
 class MetroDialogBox : public winui::Popups::IUICommandInvokedHandler {
  public:
   struct DialogBoxInfo {
-    string16 title;
-    string16 content;
-    string16 button1_label;
-    string16 button2_label;
+    base::string16 title;
+    base::string16 content;
+    base::string16 button1_label;
+    base::string16 button2_label;
     base::win::MetroDialogButtonPressedHandler button1_handler;
     base::win::MetroDialogButtonPressedHandler button2_handler;
   };