Right now the only functionality supported is getting/setting via JS
and resetting when browsing cross origin.
The HTML Specification (7.11 Browsing the web) also specifies how the
name should be restored from history entries, but we don't have those
yet.
A top level browsing context is a browsing context with no parent
browsing context.
However, we considered a top level browsing context to be a browsing
context with no associated browsing context container.