[net] Add Net.UnspecResolvedIPv6 to measure if getaddrinfo resolves IPv6 addresses.

net/base/host_resolver_impl.h

Index: net/base/host_resolver_impl.h
diff --git a/net/base/host_resolver_impl.h b/net/base/host_resolver_impl.h
index d68d6abe0caf357f256c405c44dbf3970c4de628..29a6d67731f78e321be03c8da51f955bbee2646c 100644
--- a/net/base/host_resolver_impl.h
+++ b/net/base/host_resolver_impl.h
@@ -248,6 +248,8 @@ class NET_EXPORT HostResolverImpl
   // Parameters for ProcTask.
   ProcTaskParams proc_params_;
 
+  NetLog* net_log_;
+
   // Address family to use when the request doesn't specify one.
   AddressFamily default_address_family_;
 
@@ -270,11 +272,13 @@ class NET_EXPORT HostResolverImpl
   // of the IPv6 probe job are ignored.
   bool ipv6_probe_monitoring_;
 
+  // True iff ProcTask has successfully resolved a hostname known to have IPv6
+  // addresses using ADDRESS_FAMILY_UNSPECIFIED. Reset on IP address change.
+  bool resolved_known_ipv6_hostname_;
+
   // Any resolver flags that should be added to a request by default.
   HostResolverFlags additional_resolver_flags_;
 
-  NetLog* net_log_;
-
   DISALLOW_COPY_AND_ASSIGN(HostResolverImpl);
 };