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

This can happen if an accessibility notification is queued on a node,
then that node is reparented to a different document that's not attached
anywhere before the accessibility notification is fired.

BUG=257348

Review URL: https://chromiumcodereview.appspot.com/19768007

git-svn-id: svn://svn.chromium.org/blink/trunk@154542 bbb929c8-8fbe-4397-9dbb-9b2b20218538
diff --git a/Source/core/accessibility/AXObjectCache.cpp b/Source/core/accessibility/AXObjectCache.cpp
index fe32c33..08b19d0 100644
--- a/Source/core/accessibility/AXObjectCache.cpp
+++ b/Source/core/accessibility/AXObjectCache.cpp
@@ -622,6 +622,9 @@
         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