[heap] Use right BodyDescriptor for JSWeakCollection

Workes by accident when using the JSObject descriptor as it also implements
strong semantics.

BUG=chromium:651354

Review-Url: https://codereview.chromium.org/2485163003
Cr-Commit-Position: refs/heads/master@{#40842}

src/heap/objects-visiting-inl.h

@@ -84,7 +84,10 @@ void StaticNewSpaceVisitor<StaticVisitor>::Initialize() {
 
   table_.Register(kVisitFreeSpace, &VisitFreeSpace);
 
-  table_.Register(kVisitJSWeakCollection, &JSObjectVisitor::Visit);
+  table_.Register(
+      kVisitJSWeakCollection,
+      &FlexibleBodyVisitor<StaticVisitor, JSWeakCollection::BodyDescriptor,
+                           int>::Visit);
 
   table_.Register(kVisitJSRegExp, &JSObjectVisitor::Visit);