Add base:: to string16s in win8/.

win8/metro_driver/toast_notification_handler.h

Index: win8/metro_driver/toast_notification_handler.h
diff --git a/win8/metro_driver/toast_notification_handler.h b/win8/metro_driver/toast_notification_handler.h
index 8f3587dc75ff63ef36f77f230e724a15c87f4325..f30aee810796a685f5c20d1c5eeea7fa97b7a1ef 100644
--- a/win8/metro_driver/toast_notification_handler.h
+++ b/win8/metro_driver/toast_notification_handler.h
@@ -17,12 +17,12 @@ class ToastNotificationHandler {
   struct DesktopNotification {
     std::string origin_url;
     std::string icon_url;
-    string16 title;
-    string16 body;
-    string16 display_source;
+    base::string16 title;
+    base::string16 body;
+    base::string16 display_source;
     std::string id;
     base::win::MetroNotificationClickedHandler notification_handler;
-    string16 notification_context;
+    base::string16 notification_context;
 
     DesktopNotification(const char* notification_origin,
                         const char* notification_icon,