Handle overlapping CrossThreadPersistent<> releases.

third_party/WebKit/Source/platform/heap/PersistentNode.cpp

Index: third_party/WebKit/Source/platform/heap/PersistentNode.cpp
diff --git a/third_party/WebKit/Source/platform/heap/PersistentNode.cpp b/third_party/WebKit/Source/platform/heap/PersistentNode.cpp
index 563f0fd7844b861ff5180cb20983623f600bd929..2c512958fed0f56d3fa88e42fe2b6710c21e5b8d 100644
--- a/third_party/WebKit/Source/platform/heap/PersistentNode.cpp
+++ b/third_party/WebKit/Source/platform/heap/PersistentNode.cpp
@@ -159,8 +159,10 @@ void CrossThreadPersistentRegion::prepareForThreadStateTermination(ThreadState*
             // but not invalidate its CrossThreadPersistent<>s.
             if (page->orphaned())
                 continue;
-            if (page->arena()->getThreadState() == threadState)
+            if (page->arena()->getThreadState() == threadState) {
                 persistent->clear();
+                ASSERT(slots->m_slot[i].isUnused());
+            }
         }
         slots = slots->m_next;
     }