Improve performance of proxy resolver by tracing DNS dependencies.

net/data/proxy_resolver_v8_tracing_unittest/global_sideffects4.js

+var g_iteration = 0;
+
+function FindProxyForURL(url, host) {
+  g_iteration++;
+
+  for (var i = 0; i < g_iteration; ++i) {
+    myIpAddress();
+  }
+
+  var result = '' + dnsResolve('host' + g_iteration);
+  result += g_iteration;
+
+  alert('iteration: ' + g_iteration);
+  return 'PROXY ' + result + ':34';
+}