Completely clear weak persistent references.

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

Index: third_party/WebKit/Source/platform/heap/Persistent.h
diff --git a/third_party/WebKit/Source/platform/heap/Persistent.h b/third_party/WebKit/Source/platform/heap/Persistent.h
index cf48b4999cb220e2d1de7bad996087414fd7474d..b3cfa449af04c7eb66b9f4e7f90f83f8028ef768 100644
--- a/third_party/WebKit/Source/platform/heap/Persistent.h
+++ b/third_party/WebKit/Source/platform/heap/Persistent.h
@@ -182,11 +182,10 @@ private:
     {
         static_assert(sizeof(T), "T must be fully defined");
         static_assert(IsGarbageCollectedType<T>::value, "T needs to be a garbage collected object");
-        if (weaknessConfiguration == WeakPersistentConfiguration) {
-            visitor->registerWeakCell(&m_raw);
-        } else {
+        if (weaknessConfiguration == WeakPersistentConfiguration)
+            visitor->registerWeakMembers(this, m_raw, handleWeakPersistent);
+        else
             visitor->mark(m_raw);
-        }
     }
 
     NO_LAZY_SWEEP_SANITIZE_ADDRESS
@@ -196,7 +195,7 @@ private:
         if (!m_raw || isHashTableDeletedValue())
             return;
 
-        TraceCallback traceCallback = TraceMethodDelegate<PersistentBase<T, weaknessConfiguration, crossThreadnessConfiguration>, &PersistentBase<T, weaknessConfiguration, crossThreadnessConfiguration>::tracePersistent>::trampoline;
+        TraceCallback traceCallback = TraceMethodDelegate<PersistentBase, &PersistentBase::tracePersistent>::trampoline;
         if (crossThreadnessConfiguration == CrossThreadPersistentConfiguration) {
             ProcessHeap::crossThreadPersistentRegion().allocatePersistentNode(m_persistentNode, this, traceCallback);
             return;
@@ -245,6 +244,14 @@ private:
 #endif
     }
 
+    static void handleWeakPersistent(Visitor* self, void* object)
+    {
+        using Base = PersistentBase<typename std::remove_const<T>::type, weaknessConfiguration, crossThreadnessConfiguration>;
+        Base* persistent = reinterpret_cast<Base*>(object);
+        if (persistent->get() && !ObjectAliveTrait<T>::isHeapObjectAlive(persistent->get()))
+            persistent->clear();
+    }
+
     // m_raw is accessed most, so put it at the first field.
     T* m_raw;
     PersistentNode* m_persistentNode = nullptr;