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 isn't actually required to make the suggestions work, but since this was obviously wrong, I fixed it.