Remove |was_fetched_via_proxy| from HttpResponseInfo object.

Currently, HttpResponseInfo stores ProxyServer as a net::HostPortPair object. That makes it necessary to include a separate boolean |was_fetched_via_proxy| to distinguish between the case where the resource was fetched through a DIRECT proxy vs. an invalid proxy.

Once https://bugs.chromium.org/p/chromium/issues/detail?id=646197 is fixed, HttpResponseInfo will store ProxyServer as a net::ProxyServer object. That will make it unnecessary to include the boolean variable |was_fetched_via_proxy|, and so it should be removed.