Add a new GetInstance() method for singleton classes used in chrome/browser files.

chrome/browser/gtk/browser_window_gtk.cc

Index: chrome/browser/gtk/browser_window_gtk.cc
diff --git a/chrome/browser/gtk/browser_window_gtk.cc b/chrome/browser/gtk/browser_window_gtk.cc
index 4f26fbddbba56e32b7ab44ca180292ef0e370744..f0462e58174f89d9876cd4ae1a9f6e01bd72be9c 100644
--- a/chrome/browser/gtk/browser_window_gtk.cc
+++ b/chrome/browser/gtk/browser_window_gtk.cc
@@ -1204,8 +1204,8 @@ void BrowserWindowGtk::ActiveWindowChanged(GdkWindow* active_window) {
   if (is_active && changed) {
     // If there's an app modal dialog (e.g., JS alert), try to redirect
     // the user's attention to the window owning the dialog.
-    if (Singleton<AppModalDialogQueue>()->HasActiveDialog()) {
-      Singleton<AppModalDialogQueue>()->ActivateModalDialog();
+    if (AppModalDialogQueue::GetInstance()->HasActiveDialog()) {
+      AppModalDialogQueue::GetInstance()->ActivateModalDialog();
       return;
     }
   }