Add login prompt dialog support for GTK content shell.

content/shell/shell_login_dialog.h

Index: content/shell/shell_login_dialog.h
diff --git a/content/shell/shell_login_dialog.h b/content/shell/shell_login_dialog.h
index 2d1436331aad85bbdd721af8fef9fd061571f247..9f4495cd40b478b0863cbbc244d2f68ddb5dcab7 100644
--- a/content/shell/shell_login_dialog.h
+++ b/content/shell/shell_login_dialog.h
@@ -9,6 +9,10 @@
 #include "base/string16.h"
 #include "content/public/browser/resource_dispatcher_host_login_delegate.h"
 
+#if defined(TOOLKIT_GTK)
+#include "ui/base/gtk/gtk_signal.h"
+#endif
+
 #if defined(OS_MACOSX)
 #if __OBJC__
 @class ShellLoginDialogHelper;
@@ -80,6 +84,11 @@ class ShellLoginDialog : public ResourceDispatcherHostLoginDelegate {
 #if defined(OS_MACOSX)
   // Threading: UI thread.
   ShellLoginDialogHelper* helper_;  // owned
+#elif defined(TOOLKIT_GTK)
+  GtkWidget* username_entry_;
+  GtkWidget* password_entry_;
+  GtkWidget* root_;
+  CHROMEGTK_CALLBACK_1(ShellLoginDialog, void, OnResponse, int);
 #endif
 };