Remove unused Net.IPv6ConnectDuration histogram.

This is one of the top 20 histogram by used data
size, but is unowned (thus doesn't show up on
the dashboard by default) and the data for it
has 99% of of samples fall under 3ms.

BUG=387734

Review URL: https://codereview.chromium.org/836233002

Cr-Commit-Position: refs/heads/master@{#310170}

net/dns/host_resolver_impl.cc

@@ -2155,7 +2155,6 @@ HostResolverImpl::Key HostResolverImpl::GetEffectiveKeyForRequest(
 
   if (info.address_family() == ADDRESS_FAMILY_UNSPECIFIED) {
     if (probe_ipv6_support_ && !use_local_ipv6_) {
-      base::TimeTicks start_time = base::TimeTicks::Now();
       // Google DNS address.
       const uint8 kIPv6Address[] =
           { 0x20, 0x01, 0x48, 0x60, 0x48, 0x60, 0x00, 0x00,
@@ -2171,8 +2170,6 @@ HostResolverImpl::Key HostResolverImpl::GetEffectiveKeyForRequest(
       if (rv6)
         net_log.AddEvent(NetLog::TYPE_HOST_RESOLVER_IMPL_IPV6_SUPPORTED);
 
-      UMA_HISTOGRAM_TIMES("Net.IPv6ConnectDuration",
-                          base::TimeTicks::Now() - start_time);
       if (rv6) {
         UMA_HISTOGRAM_BOOLEAN("Net.IPv6ConnectSuccessMatch",
             default_address_family_ == ADDRESS_FAMILY_UNSPECIFIED);

tools/metrics/histograms/histograms.xml

@@ -16968,6 +16968,9 @@ Therefore, the affected-histogram name has to have at least one dot in it.
 </histogram>
 
 <histogram name="Net.IPv6ConnectDuration" units="milliseconds">
+  <obsolete>
+    Deprecated 1/2015. No longer tracked.
+  </obsolete>
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>Duration of time spent during the UDP-connect IPv6 probe.</summary>
 </histogram>