src: zero-initialize data that are copied into the snapshot

To prevent padding from making the snapshot unreproducible,
zero-initialize the data that are copied into the snapshot
so that the padding copied are all zeros. This is better
than enlarging the enums to align the fields since it doesn't
make the snapshot bigger than necessary, and it removes the
need of using static assertions to ensure alignment.

Author: joyeecheung

src/node_snapshotable.cc

@@ -1416,6 +1416,7 @@ StartupData SerializeNodeContextInternalFields(Local<Object> holder,
   if (index == BaseObject::kEmbedderType) {
     int size = sizeof(EmbedderTypeInfo);
     char* data = new char[size];
+    memset(data, 0, size);  // Make the padding reproducible.
     // We need to use placement new because V8 calls delete[] on the returned
     // data.
     // TODO(joyeecheung): support cppgc objects.

src/node_snapshotable.h

@@ -47,6 +47,7 @@ struct InternalFieldInfoBase {
                       std::is_same_v<InternalFieldInfoBase, T>,
                   "Can only accept InternalFieldInfoBase subclasses");
     void* buf = ::operator new[](sizeof(T));
+    memset(buf, 0, sizeof(T));  // Make the padding reproducible.
     T* result = new (buf) T;
     result->type = type;
     result->length = sizeof(T);