Enable DNS-over-HTTPS upgrades when possible (fixes brave/brave-brows…

…er#1864)

This will not change a user's DNS provider. Instead, we will only
upgrade to DoH if the OS resolver is known to support DoH (e.g.
Cloudflare or Google).

This essentially enables for all users the DoH experiment that
Chrome rolled out to 1% of its users:

https://docs.google.com/document/d/15Ss0OaJeb-T3g2RMwgikHvsC0CPKd-MLeGeetv1wYY4/edit#

app/brave_main_delegate.cc

@@ -160,6 +160,7 @@ bool BraveMainDelegate::BasicStartupComplete(int* exit_code) {
     // this feature.
       features::kWebUIDarkMode.name,
       omnibox::kSimplifyHttpsIndicator.name,
+      features::kDnsOverHttps.name,
   };
 
   // Disabled features.