Handle cross-thread weak persistents during global weak processing.

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

Index: third_party/WebKit/Source/platform/heap/MarkingVisitorImpl.h
diff --git a/third_party/WebKit/Source/platform/heap/MarkingVisitorImpl.h b/third_party/WebKit/Source/platform/heap/MarkingVisitorImpl.h
index 03437ed92fe87fb219781f7bede8d9125a6d51b1..7e9e09fbdf617c588e61a1d76030081608dbe0d7 100644
--- a/third_party/WebKit/Source/platform/heap/MarkingVisitorImpl.h
+++ b/third_party/WebKit/Source/platform/heap/MarkingVisitorImpl.h
@@ -104,12 +104,6 @@ protected:
         return true;
     }
 
-    Derived* toDerived()
-    {
-        return static_cast<Derived*>(this);
-    }
-
-protected:
     inline void registerWeakCellWithCallback(void** cell, WeakCallback callback)
     {
         ASSERT(toDerived()->getMarkingMode() != Visitor::WeakProcessing);
@@ -119,6 +113,11 @@ protected:
         toDerived()->heap().pushGlobalWeakCallback(cell, callback);
     }
 
+    Derived* toDerived()
+    {
+        return static_cast<Derived*>(this);
+    }
+
 private:
     static void markNoTracingCallback(Visitor* visitor, void* object)
     {