Improve performance of proxy resolver by tracing DNS dependencies.

net/data/proxy_resolver_v8_tracing_unittest/error.js

Index: net/data/proxy_resolver_v8_tracing_unittest/error.js
diff --git a/net/data/proxy_resolver_v8_tracing_unittest/error.js b/net/data/proxy_resolver_v8_tracing_unittest/error.js
new file mode 100644
index 0000000000000000000000000000000000000000..83e534c28f2dbbaf3cbbb056bea47b8abb4cd04d
--- /dev/null
+++ b/net/data/proxy_resolver_v8_tracing_unittest/error.js
@@ -0,0 +1,8 @@
+function FindProxyForURL(url, host) {
+  if (host == 'throw-an-error') {
+    alert('Prepare to DIE!');
+    var x = null;
+    return x.split('-');  // Throws exception.
+  }
+  return "PROXY i-approve-this-message:42";
+}