Make the inspector search actually suggest selectors

This required to basically copy/paste `getSuggestionsForQuery` from the devtools walker actor into the chromium walker actor, adapting it so it goes via the device protocol instead.
I also took the opportunity to rewrite the function (got from 100+ lines to 60).
This seems to work well with both safari and chrome.

@campd can you take a look at this PR?
This is worth merging, but I wanted to mention that chrome's querySelectorAll is way inefficient, and always sends every match over.  We should probably use their search methods in our search field instead of qsa.

I think that would mean exposing a search method ourselves and using that fro, the front end.

But we don't need to do that right this second.