[net] Add AsyncDns field trial.

net/base/host_resolver_impl.cc

Index: net/base/host_resolver_impl.cc
diff --git a/net/base/host_resolver_impl.cc b/net/base/host_resolver_impl.cc
index ce6f1264f40da6914f6ecbc9a48d997d665ffe2a..2803a3cfb88a29e0df361daa93e71cf54c0b6c0a 100644
--- a/net/base/host_resolver_impl.cc
+++ b/net/base/host_resolver_impl.cc
@@ -467,6 +467,18 @@ HostResolver* CreateAsyncHostResolver(size_t max_concurrent_resolves,
                             net_log);
 }
 
+bool ConfigureAsyncDnsFieldTrial() {
+  const base::FieldTrial::Probability kAsyncDnsDivisor = 100;

[cbentzel, 2012/08/20 11:45:56]

Perhaps restrict enabled_probability to non-0 values for canary+dev channels
only to be safe.

[szym, 2012/08/20 17:54:21]

Good idea although net cannot depend on chrome/ and only chrome/ knows channels.
I could just disable the field trial in io_thread.cc for the other channels, but
to have the flexibility of per-channel probability, I'll have to move this to
chrome/browser/net/async_dns_field_trial.{h,cc}.

[cbentzel, 2012/08/21 10:50:53]

Yes. I personally think it makes sense there - you don't want to run this field
trial for non-Chrome embedders of the network stack.

+  base::FieldTrial::Probability enabled_probability = 50;
+
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "AsyncDns", kAsyncDnsDivisor, "disabled", 2012, 9, 30, NULL));
+
+  int enabled_group = trial->AppendGroup("enabled", enabled_probability);

[cbentzel, 2012/08/21 10:50:53]

Should this be restricted to supported platforms only? What if 50% are "enabled"
in iOS or Android?

+  return trial->group() == enabled_group;
+}
+
 //-----------------------------------------------------------------------------
 
 // Holds the data for a request that could not be completed synchronously.