Don't post accessibility notification on object in detached document.

Source/core/accessibility/AXObjectCache.cpp

Index: Source/core/accessibility/AXObjectCache.cpp
diff --git a/Source/core/accessibility/AXObjectCache.cpp b/Source/core/accessibility/AXObjectCache.cpp
index dd40ea50d3a923c4f7620566ee965d10267e31e9..2cf6f6dc58c84b51f1908b757a9d46cb0eb9e0e7 100644
--- a/Source/core/accessibility/AXObjectCache.cpp
+++ b/Source/core/accessibility/AXObjectCache.cpp
@@ -622,6 +622,9 @@ void AXObjectCache::notificationPostTimerFired(Timer<AXObjectCache>*)
         if (!obj->axObjectID())
             continue;
 
+        if (!obj->axObjectCache())
+            continue;
+
 #ifndef NDEBUG
         // Make sure none of the render views are in the process of being layed out.
         // Notifications should only be sent after the renderer has finished