Purposes Extension: Change reference to StorageAccess API to reference WhatWG discussion

[jasonanovak]

In the July 23, 2018 Editor's Draft of Tracking Preference Expression (DNT)- Purposes Extension Addendum, the Introduction says:

As further security measures this value would only be settable when the registering API call is made within a user gesture, similar to the way the Webkit [StorageAPI] requestStorageAccess() is constrained, and only when the request is secure, i.e. delivered over https.

The StorageAPI citation is to a WebKit blogpost; it may be more effective to reference the WhatWG discussion standardizing the API, whatwg/html#3338.

[rvaneijk]

I agree. This is a good editorial suggestion.

[rvaneijk]

I think the issues 91 and 92 can be merged since they suggest editorial changes.

[michael-oneill]

I have changed the citation