Eric is referring to extension scope for modifications. Essentially, when building an extension, one must consider "feature availability" with "can this be justified, at the risk of someone potentially misusing it" for regular users. The address bar is one of these components which are immutable for modifications, aside from colour and textile modifications to icons by design.
Specifically, the fullscreen API within Chrome is not exposed to extensions as without extensive restrictions, it would possible to break the browser if someone wished to because of how low level the API is internally within the browser (and the extension is hooked in).
Hope this helps.
Cheers,
Decklin