Fix a crash when processing an invalid PAC script.

net/data/proxy_resolver_v8_unittest/exception_findproxyforurl_during_resolve.js

+// This PAC script will throw an exception when accessing the "FindProxyForURL"
+// the *second* time it is accessed.
+
+function actualFindProxyForURL(url, host) {
+  return "DIRECT";
+}
+
+var counter = 2;
+
+function FindProxyForURLGetter() {

[mmenke, 2015/02/20 22:15:05]

nit:  f should be lowercase, I believe.

+  counter--;
+  if (counter <= 0)
+    throw "crash!";
+  return actualFindProxyForURL;
+}
+
+Object.defineProperty(this, "FindProxyForURL", {get: FindProxyForURLGetter});