Support "await a stable state" and "provide a stable state"

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 189f09e2622f96b38ed90d0ce34e5ecc5b927c68..46496ade804b9b91b75333619e925da1504fbf6c 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -137,6 +137,7 @@
 #include "core/html/HTMLStyleElement.h"
 #include "core/html/HTMLTitleElement.h"
 #include "core/html/PluginDocument.h"
+#include "core/html/StableState.h"
 #include "core/html/forms/FormController.h"
 #include "core/html/parser/HTMLDocumentParser.h"
 #include "core/html/parser/HTMLParserIdioms.h"
@@ -2516,6 +2517,9 @@ bool Document::dispatchBeforeUnloadEvent(Chrome& chrome, bool& didAllowNavigatio
         return true;
     }
 
+    // http://whatwg.org/html#pause
+    StableState::provide();
+
     String text = beforeUnloadEvent->returnValue();
     if (chrome.runBeforeUnloadConfirmPanel(text, m_frame)) {
         didAllowNavigation = true;