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

chrome/common/render_messages_internal.h

Index: chrome/common/render_messages_internal.h
diff --git a/chrome/common/render_messages_internal.h b/chrome/common/render_messages_internal.h
index 23b997d7e8eb3eeb9a7626504977f78aa2617ba7..381255292d8988218945d4c6c8ca9968e15d7e6c 100644
--- a/chrome/common/render_messages_internal.h
+++ b/chrome/common/render_messages_internal.h
@@ -1334,11 +1334,12 @@ IPC_MESSAGE_ROUTED2(ViewHostMsg_DownloadUrl,
 IPC_MESSAGE_ROUTED1(ViewHostMsg_GoToEntryAtOffset,
                     int /* offset (from current) of history item to get */)
 
-IPC_SYNC_MESSAGE_ROUTED4_2(ViewHostMsg_RunJavaScriptMessage,
+IPC_SYNC_MESSAGE_ROUTED5_2(ViewHostMsg_RunJavaScriptMessage,
                            std::wstring /* in - alert message */,
                            std::wstring /* in - default prompt */,
                            GURL         /* in - originating page URL */,
                            int          /* in - dialog flags */,
+                           bool         /* in - unload handler being run? */,
                            bool         /* out - success */,
                            std::wstring /* out - prompt field */)