This object is available as `window.internals` (or just `internals`) and
is only accessible while running in "test mode".
This first version only has one API: gc(), which triggers a garbage
collection immediately.
In the future, we can add more APIs here to help us test parts of the
engine that are hard or impossible to reach via public web APIs.
For example, Document.getSelection returns Selection, which is in the
Selection namespace.
Namespaces.h has Linus' copyright since he changed the "is_one_of" list
to an Array.
