This makes it more similar to other handle types (like PersistentBase),
by simply storing an i::Object** cast to T*. This means that it is not
necessary to look up the handle in the eternal handles table to access
the underlying value.

Like the built-in roots (null, etc.), an eternal handle can never be
destroyed, so we don't even need to allocate a separate local handle.
Instead, the Local<T> can point directly at the eternal reference.
This makes Eternal<T>::Get trivial.

Review-Url: https://codereview.chromium.org/2751263003
Cr-Commit-Position: refs/heads/master@{#43912}