[net/dns] Test IPv6 support via UDP connect (measurement)

[net/dns] Test IPv6 support via UDP connect (measurement)

Network interface enumeration is expensive and requires absolutely no false negatives
in network change detection. In contrast bionic (Android libc) connects a UDP socket
to an IPv6 address which confirms that the kernel can route to such address.

This CL adds the UDP connect test to measure its performance compared to the probe.

BUG=223876
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=193421

[mmenke, 2013-04-10 16:03:25]

LGTM

[commit-bot: I haz the power, 2013-04-10 16:05:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/szym@chromium.org/13812002/2001

[commit-bot: I haz the power, 2013-04-10 18:10:52]

Change committed as 193421

[lorenzo, 2013-04-11 07:28:14]

https://chromiumcodereview.appspot.com/13812002/diff/2001/net/dns/host_resolv...
File net/dns/host_resolver_impl.cc (right):

https://chromiumcodereview.appspot.com/13812002/diff/2001/net/dns/host_resolv...
net/dns/host_resolver_impl.cc:180: return socket->Connect(IPEndPoint(dest, 80));
Do you need to check that the address is a global address as well? That's what
the existing IPv6 probe does I think.

It's possible that some operating systems (perhaps OS X; Linux doesn't do this),
will try to connect to a global address even if they only have a link-local
address (which is very unlikely to work). On these systems, the connection probe
could return success even if there was no global address present on the machine.

If that's the case, you can call getsockname() to get the IP address the OS
picked for the connection and see if it's a global address.