Issue 594601  documents a subset of this.

There is some need to modify DNS results that Cronet uses for requests:
1. remove bad results caused by DNS spoofing
2. replace results with specific entries for testing purposes
    a. for unit testing we need to direct the requests to the test server
    b. for performance/smoke testing we need to direct the requests to a particular server

There could be other needs that could require having Cronet avoid doing DNS resolution at all:
1. in the interest of performance if a trusted server pointed us at another URL and also gave us the DNS results for the host in that URL, then we should skip Cronet's normal DNS resolution procedure entirely
2. in a multi-network environment we might want to do DNS via some other mechanism to ensure we get DNS results only on some particular network (though this is likely something that Cronet will handle itself).