Add IPv6 probe support for Windows

This is meant to test to see if AI_ADDRCONFIG is doing its
job in windows, or if we have to do the scans when there is
a network change.

Also switch to always doing probe support on Mac/Linux platforms.
There appears to be (on aggregate) around a 40% degradation of
DNS resolution times on the Mac, and about 17% degration on
Linux, when we don't probe.  It is likely that a few users
are greatly impacted by this, and are able to move the average
of the whole user population.


BUG=12754
BUG=25680
r=wtc
Review URL: http://codereview.chromium.org/1210003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@42544 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 6 insertions, 2 deletions

diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index d2ce04e..83d81b6 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -40,11 +40,13 @@ net::HostResolver* CreateGlobalHostResolver(
           global_host_resolver->GetAsHostResolverImpl();
       if (host_resolver_impl != NULL) {
         // (optionally) Use probe to decide if support is warranted.
+        bool use_ipv6_probe = true;
 
+#if defined(OS_WIN)
         // Measure impact of probing to allow IPv6.
         // Some users report confused OS handling of IPv6, leading to large
         // latency.  If we can show that IPv6 is not supported, then disabliing
-        // it will work around such problems.
+        // it will work around such problems. This is the test of the probe.
         const FieldTrial::Probability kDivisor = 100;
         const FieldTrial::Probability kProbability = 50;  // 50% probability.
         FieldTrial* trial = new FieldTrial("IPv6_Probe", kDivisor);
@@ -52,7 +54,9 @@ net::HostResolver* CreateGlobalHostResolver(
                                             kProbability);
         trial->AppendGroup("_IPv6_probe_done",
                            FieldTrial::kAllRemainingProbability);
-        bool use_ipv6_probe = (trial->group() != skip_group);
+        use_ipv6_probe = (trial->group() != skip_group);
+#endif
+
         if (use_ipv6_probe)
           host_resolver_impl->ProbeIPv6Support();
       }