ACTION-625: Clarify that chrome refers to primary and secondary UI

Since I didn't see a specific edit in the minutes from the relevant  
meeting, I've discharged this action item by adding the following text  
to section 4.2.1:

We occasionally use the term [Definition: chrome] to refer to the  
representation through which the user interacts with the user agent  
itself, as distinct from the web content accessed. This includes  
primary and secondary user interface.

The phrase used to define "chrome" comes from wsc-usecases.

Regards,
--
Thomas Roessler, W3C  <tlr@w3.org>