Shut down all threads before the main thread shuts down

third_party/WebKit/Source/platform/heap/ThreadState.h

Index: third_party/WebKit/Source/platform/heap/ThreadState.h
diff --git a/third_party/WebKit/Source/platform/heap/ThreadState.h b/third_party/WebKit/Source/platform/heap/ThreadState.h
index 885b66d873b71857ada5e89ac651a984359bd560..37f1375b8ae390daf912e446f7b98598605457aa 100644
--- a/third_party/WebKit/Source/platform/heap/ThreadState.h
+++ b/third_party/WebKit/Source/platform/heap/ThreadState.h
@@ -432,6 +432,11 @@ public:
         m_isolate = isolate;
         m_traceDOMWrappers = traceDOMWrappers;
     }
+    void unregisterTraceDOMWrappers()
+    {
+        m_isolate = nullptr;
+        m_traceDOMWrappers = nullptr;
+    }
 
     // By entering a gc-forbidden scope, conservative GCs will not
     // be allowed while handling an out-of-line allocation request.