Add support for ProxyResolverErrorObserver to ProxyResolverMojo.

net/interfaces/proxy_resolver_service.mojom

Index: net/interfaces/proxy_resolver_service.mojom
diff --git a/net/interfaces/proxy_resolver_service.mojom b/net/interfaces/proxy_resolver_service.mojom
index 57ab12137693af4247aab88c1dbd1b3aeabe8ea9..68f798e8e8964dc85756a169757c7f407b75bb4e 100644
--- a/net/interfaces/proxy_resolver_service.mojom
+++ b/net/interfaces/proxy_resolver_service.mojom
@@ -42,11 +42,16 @@ interface ProxyResolverRequestClient {
   LoadStateChanged(int32 load_state);
 };
 
+interface ProxyResolverErrorObserver {
+  OnPacScriptError(int32 line_number, string error);
+};
+
 interface ProxyResolverFactory {
-  // TODO(amistry): Add NetLog and ProxyResolverErrorObserver.
+  // TODO(amistry): Add NetLog.
   CreateResolver(string pac_script,
                  ProxyResolver& resolver,
                  HostResolver host_resolver,
+                 ProxyResolverErrorObserver? error_observer,
                  ProxyResolverFactoryRequestClient client);
 };