[Cleanup] Remove StringPrintf from global namespace

ash/system/web_notification/web_notification_tray_unittest.cc

Index: ash/system/web_notification/web_notification_tray_unittest.cc
diff --git a/ash/system/web_notification/web_notification_tray_unittest.cc b/ash/system/web_notification/web_notification_tray_unittest.cc
index a53f970c5e1dc74d81494deb69243606876d6dff..d5aba2d8fbdecdbc065bf81bedcc7cc0946bc69f 100644
--- a/ash/system/web_notification/web_notification_tray_unittest.cc
+++ b/ash/system/web_notification/web_notification_tray_unittest.cc
@@ -196,7 +196,7 @@ TEST_F(WebNotificationTrayTest, ManyMessageCenterNotifications) {
   size_t notifications_to_add =
       NotificationList::kMaxVisibleMessageCenterNotifications + 1;
   for (size_t i = 0; i < notifications_to_add; ++i) {
-    std::string id = StringPrintf("test_id%d", static_cast<int>(i));
+    std::string id = base::StringPrintf("test_id%d", static_cast<int>(i));
     delegate->AddNotification(tray, id);
   }
   bool shown = tray->message_center_tray_->ShowMessageCenterBubble();
@@ -217,7 +217,7 @@ TEST_F(WebNotificationTrayTest, ManyPopupNotifications) {
   size_t notifications_to_add =
       NotificationList::kMaxVisiblePopupNotifications + 1;
   for (size_t i = 0; i < notifications_to_add; ++i) {
-    std::string id = StringPrintf("test_id%d", static_cast<int>(i));
+    std::string id = base::StringPrintf("test_id%d", static_cast<int>(i));
     delegate->AddNotification(tray, id);
   }
   // Hide and reshow the bubble so that it is updated immediately, not delayed.