Ignore JavaScript messages (alert/confirm/prompt) during unload handlers.

content/browser/tab_contents/tab_contents.cc

Index: content/browser/tab_contents/tab_contents.cc
diff --git a/content/browser/tab_contents/tab_contents.cc b/content/browser/tab_contents/tab_contents.cc
index 220e7697c8a77768d31087e574619ef39fbeb6aa..e4fb6180cbbe9ce4a7b4d4b5b58af7c621baed1a 100644
--- a/content/browser/tab_contents/tab_contents.cc
+++ b/content/browser/tab_contents/tab_contents.cc
@@ -2406,6 +2406,7 @@ void TabContents::RunJavaScriptMessage(
     const std::wstring& default_prompt,
     const GURL& frame_url,
     const int flags,
+    const bool unload_handler_being_run,
     IPC::Message* reply_msg,
     bool* did_suppress_message) {
   // Suppress javascript messages when requested and when inside a constrained
@@ -2414,9 +2415,11 @@ void TabContents::RunJavaScriptMessage(
   // Also suppress messages when showing an interstitial. The interstitial is
   // shown over the previous page, we don't want the hidden page dialogs to
   // interfere with the interstitial.
+  // Also suppress messages during beforeunload/unload event handlers.
   bool suppress_this_message =
       suppress_javascript_messages_ ||
       showing_interstitial_page() ||
+      unload_handler_being_run ||
       (delegate() && delegate()->ShouldSuppressDialogs());
   if (delegate())
     suppress_this_message |=