Provide a clear way to hook into "document becomes visible/hidden"

[siusin]

Quoted from @mounirlamouri 's request in http://lists.w3.org/Archives/Public/public-web-perf/2014Sep/0034.html, also see screen-orientation #77.

The specification has "now visible algorithm" and "now hidden algorithm"
but misses extension points at the end of those algorithms. It would be
great to add such extension points so other specifications could cleanly
define behaviour when the document visibility state changes.

The ref section in the SO spec

Whenever a document becomes visible per [PAGE-VISIBILITY], in other words after the now visible algorithm is run, the user agent MUST run the following substeps...

[toddreifsteck]

Per discussion on WG call on 8/20, review for a way to add this hook in L2.