Handle cross-thread weak persistents during global weak processing.

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

Index: third_party/WebKit/Source/platform/heap/Visitor.h
diff --git a/third_party/WebKit/Source/platform/heap/Visitor.h b/third_party/WebKit/Source/platform/heap/Visitor.h
index 97a660d3bc85b648b21b8955f9f422bc06b88222..9873523d64c1efc0002dcd7c92b815fccfac82c6 100644
--- a/third_party/WebKit/Source/platform/heap/Visitor.h
+++ b/third_party/WebKit/Source/platform/heap/Visitor.h
@@ -318,13 +318,13 @@ public:
 
     virtual bool ensureMarked(const void*) = 0;
 
+    virtual void registerWeakCellWithCallback(void**, WeakCallback) = 0;
+
     inline MarkingMode getMarkingMode() const { return m_markingMode; }
 
 protected:
     Visitor(ThreadState*, MarkingMode);
 
-    virtual void registerWeakCellWithCallback(void**, WeakCallback) = 0;
-
 private:
     static Visitor* fromHelper(VisitorHelper<Visitor>* helper) { return static_cast<Visitor*>(helper); }