Handle primitive values in WeakMap when taking snapshot

Summary:
Scan WeakMapEntrySlots to visit (possible) primitive values with
PrimitiveNodeAcceptor, so that we can successfully add edges to them in
WeakMap.

Reviewed By: neildhar

Differential Revision: D55450685

fbshipit-source-id: 67bb29484ee94e5789fbd955f94867a1ec783e82

lib/VM/GCBase.cpp

@@ -540,6 +540,11 @@ void GCBase::createSnapshot(GC &gc, llvh::raw_ostream &os) {
             stackTracesTreeNode ? stackTracesTreeNode->id : 0);
       };
   gc.forAllObjs(snapshotForObject);
+  // Scan all WeakMapEntrySlot so that PrimitiveNodeAcceptor won't miss
+  // primitives stored as WeakMap values.
+  weakMapEntrySlots_.forEach([&primitiveAcceptor](WeakMapEntrySlot &slot) {
+    primitiveAcceptor.accept(slot.mappedValue);
+  });
   // Write the singleton number nodes into the snapshot.
   primitiveAcceptor.writeAllNodes();
   snap.endSection(HeapSnapshot::Section::Nodes);

lib/VM/JSWeakMapImpl.cpp

@@ -160,14 +160,6 @@ void JSWeakMapImplBase::_snapshotAddEdgesImpl(
         gc.getObjectID(key.getKeyNonNull(gc.getPointerBase(), gc)));
 
     auto mappedValue = key.getMappedValue(gc);
-    // TODO(T175014649): nodes for numbers may not exist since they are not seen
-    // by PrimitiveNodeAcceptor. We can't simply add them in
-    // _snapshotAddNodesImpl() either, because PrimitiveNodeAcceptor may see the
-    // same number in a heap object and the assertion of not writing duplicate
-    // node will fail.
-    if (mappedValue.isNumber()) {
-      continue;
-    }
     if (auto id = gc.getSnapshotID(mappedValue)) {
       snap.addNamedEdge(
           HeapSnapshot::EdgeType::Internal,