Merge 50561 - Fix a crash when we try to close a js dialog that wasn't shown....

chrome/browser/app_modal_dialog_gtk.cc

Index: chrome/browser/app_modal_dialog_gtk.cc
===================================================================
--- chrome/browser/app_modal_dialog_gtk.cc	(revision 51020)
+++ chrome/browser/app_modal_dialog_gtk.cc	(working copy)
@@ -31,17 +31,21 @@
 }
 
 void AppModalDialog::ActivateModalDialog() {
+  DCHECK(dialog_);
   gtk_window_present(GTK_WINDOW(dialog_));
 }
 
 void AppModalDialog::CloseModalDialog() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_DELETE_EVENT);
 }
 
 void AppModalDialog::AcceptWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_OK);
 }
 
 void AppModalDialog::CancelWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_CANCEL);
 }