Get related documents from the browser page

This is just an initial iteration to start a discussion on the approach. It now provides an on-demand mechanism to retrieve URLs of external JS and Stylesheets files loaded by the browser by adding Document.getRelated operation to LiveDevProtocol. It could eventually be evolved to support separated calls (JS files, Stylesheets, embedded scripts, etc.). The main restriction probably is the lack of events to track added/removed files which would allow retrieving related documents after connection and then track the changes. 

@shahabl, please take a look to see if this can be an alternative (which should be evolved) to address the cases you already have in mind. It could probably allows us to avoid adding logic on the server-side to identify relationships that the browser already solves for us in this case.