Definition of "visit" (ACTION-303)

A number of places in the document have a notion of "the site a user is
visiting" (largely in relation to determining whether something is a first
or third party). I believe it's important for us to have a concrete,
unambiguous definition of this.

What I propose is to use the definition from the HTML spec. (
http://dev.w3.org/html5/spec/single-page.html?utm_source=dlvr.it&utm_medium=feed#windows
)

A user "visits" a given URI when the user takes action (such as typing that
URI into an address bar, clicking a link to that URI on another website, or
clicking a link to that URI from an external program that opens a web user
agent) that results in a _browsing context_ whose _session history_
contains a _Document_ with an _address_ matching the given URI.

This is about as concrete and unambiguous as I can make it. By definition,
this also resolves ACTION-304 in that the user would "visit" any redirects
that were involved in "visit"ing anything else.