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

Source/core/frame/DOMWindow.cpp

Index: Source/core/frame/DOMWindow.cpp
diff --git a/Source/core/frame/DOMWindow.cpp b/Source/core/frame/DOMWindow.cpp
index 28661b8e793fe0cf85b0d9e0df6f8988365e1dd5..8953860447cf75b23567b10e516767539571cd42 100644
--- a/Source/core/frame/DOMWindow.cpp
+++ b/Source/core/frame/DOMWindow.cpp
@@ -70,6 +70,7 @@
 #include "core/frame/Screen.h"
 #include "core/frame/Settings.h"
 #include "core/html/HTMLFrameOwnerElement.h"
+#include "core/html/StableState.h"
 #include "core/inspector/InspectorInstrumentation.h"
 #include "core/inspector/ScriptCallStack.h"
 #include "core/loader/DocumentLoader.h"
@@ -1012,6 +1013,10 @@ void DOMWindow::print()
         return;
     }
     m_shouldPrintWhenFinishedLoading = false;
+
+    // http://whatwg.org/html#pause
+    StableState::provide();
+
     host->chrome().print(m_frame);
 }
 
@@ -1033,6 +1038,9 @@ void DOMWindow::alert(const String& message)
     if (!host)
         return;
 
+    // http://whatwg.org/html#pause
+    StableState::provide();
+
     host->chrome().runJavaScriptAlert(m_frame, message);
 }
 
@@ -1047,6 +1055,9 @@ bool DOMWindow::confirm(const String& message)
     if (!host)
         return false;
 
+    // http://whatwg.org/html#pause
+    StableState::provide();
+
     return host->chrome().runJavaScriptConfirm(m_frame, message);
 }
 
@@ -1061,6 +1072,9 @@ String DOMWindow::prompt(const String& message, const String& defaultValue)
     if (!host)
         return String();
 
+    // http://whatwg.org/html#pause
+    StableState::provide();
+
     String returnValue;
     if (host->chrome().runJavaScriptPrompt(m_frame, message, defaultValue, returnValue))
         return returnValue;