Fix a Windows crash bug with javascript alerts from extension popups.

chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc

Index: chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc
diff --git a/chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc b/chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc
index dd6732f3565c1aba96b54c68ec901aee7f2e9ddd..e5b7c767360175a42f887c8b97edb2b4167a3787 100644
--- a/chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc
+++ b/chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc
@@ -18,10 +18,12 @@ AppModalDialog::AppModalDialog(WebContents* web_contents, const string16& title)
     : valid_(true),
       native_dialog_(NULL),
       title_(title),
-      web_contents_(web_contents) {
+      web_contents_(web_contents),
+      completed_(false) {
 }
 
 AppModalDialog::~AppModalDialog() {
+  CompleteDialog();
 }
 
 void AppModalDialog::ShowModalDialog() {
@@ -62,5 +64,8 @@ void AppModalDialog::CloseModalDialog() {
 }
 
 void AppModalDialog::CompleteDialog() {
-  AppModalDialogQueue::GetInstance()->ShowNextDialog();
+  if (!completed_) {
+    completed_ = true;
+    AppModalDialogQueue::GetInstance()->ShowNextDialog();
+  }
 }