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 d8fd5994be5eed3527b3ca90ef96f93c0ee21d03..199a2116ae8376e979c6e15d32e65f4e2aba6175 100644
--- a/net/interfaces/proxy_resolver_service.mojom
+++ b/net/interfaces/proxy_resolver_service.mojom
@@ -44,8 +44,13 @@ 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(ProxyResolver& resolver,
-                 HostResolver host_resolver);
+                 HostResolver host_resolver,
+                 ProxyResolverErrorObserver? error_observer);
 };