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

chrome/browser/ui/app_modal_dialogs/message_box_handler.cc

Index: chrome/browser/ui/app_modal_dialogs/message_box_handler.cc
diff --git a/chrome/browser/ui/app_modal_dialogs/message_box_handler.cc b/chrome/browser/ui/app_modal_dialogs/message_box_handler.cc
index 490117ca9997ef19a4d4145c35d3273d3754023f..0aba84b083bc4ba7f678c3b7730b2ec2f0447657 100644
--- a/chrome/browser/ui/app_modal_dialogs/message_box_handler.cc
+++ b/chrome/browser/ui/app_modal_dialogs/message_box_handler.cc
@@ -70,7 +70,7 @@ void RunJavascriptMessageBox(Profile* profile,
                              IPC::Message* reply_msg) {
   bool is_alert = dialog_flags == MessageBoxFlags::kIsJavascriptAlert;
   std::wstring title = GetTitle(profile, is_alert, frame_url);
-  Singleton<AppModalDialogQueue>()->AddDialog(new JavaScriptAppModalDialog(
+  AppModalDialogQueue::GetInstance()->AddDialog(new JavaScriptAppModalDialog(
       delegate, title, dialog_flags, message_text, default_prompt_text,
       display_suppress_checkbox, false, reply_msg));
 }
@@ -81,7 +81,7 @@ void RunBeforeUnloadDialog(TabContents* tab_contents,
   std::wstring full_message =
       message_text + L"\n\n" +
       l10n_util::GetString(IDS_BEFOREUNLOAD_MESSAGEBOX_FOOTER);
-  Singleton<AppModalDialogQueue>()->AddDialog(new JavaScriptAppModalDialog(
+  AppModalDialogQueue::GetInstance()->AddDialog(new JavaScriptAppModalDialog(
       tab_contents, l10n_util::GetString(IDS_BEFOREUNLOAD_MESSAGEBOX_TITLE),
       MessageBoxFlags::kIsJavascriptConfirm, message_text, std::wstring(),
       false, true, reply_msg));