[net/dns] Add per-address-family histograms AsyncDNS.ResolveSuccess_FAMILY_*

[net/dns] Add per-address-family histograms AsyncDNS.ResolveSuccess_FAMILY_* and per-qtype histograms AsyncDNS.TransactionSuccess_*.

Currently observed AsyncDNS.ResolveSuccess (duration of DnsTask) is almost twice as long as AsyncDNS.TransactionSuccess (duration of DnsTransaction). This could be attributed to dual-stack resolutions which involve two DnsTransactions, but at the same time the difference between DNS.ResolveSuccess_FAMILY_IPV4 and ..._UNSPEC (duration of ProcTask) is much less pronounced suggesting that SystemDNS resolves names on dual-stack hosts differently than AsyncDNS.

This measurement will allow us to determine how much of the observed overhead of DnsTask over DnsTransaction can be attributed to dual-stack lookups.

BUG=169257
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=176137

[szym, 2013-01-09 23:18:04]

Switching reviewer from Matt to Paul with whom I have already briefly discussed
this idea.

[pauljensen, 2013-01-10 14:58:33]

lgtm.  In your description you should change "must" to "much".  Would be nice if
there was a related bug.  Might also be nice to have a histogram of the time at
line 1049 in case there's any difference in the two transaction times or to
break up AsyncDNS.TransactionSuccess by family_ but that's all up to you.

[commit-bot: I haz the power, 2013-01-10 16:32:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/szym@chromium.org/11783076/7001

[commit-bot: I haz the power, 2013-01-10 18:53:41]

Retried try job too often on linux_chromeos for step(s) browser_tests