- Wrap the initialization of the kSlot and kEmbedderType fields
  into a BaseObject::SetInternalFields() method.
- Move the tagging of kEmbedderType field into
  BaseObject::TagNodeObject()
- Add a variant of BaseObject::MakeLazilyInitializedJSTemplate()
  that only needs IsolateData.
This makes it easier to create BaseObject subclasses.

Instead of adding a reference to the ContextifyContext by using
a v8::External, we make ContextifyContext a weak BaseObject that
whose wrapper is referenced by the sandbox via a private symbol.
This makes it easier to snapshot the contexts, in addition to
reusing the BaseObject lifetime management for ContextifyContexts.

This reverts commit b9042c87b60184a37d9c8674ec8c8b29b2a8ab5e.