Use std::string instead of mojo::String in //net.

net/proxy/mojo_proxy_resolver_factory.h

Index: net/proxy/mojo_proxy_resolver_factory.h
diff --git a/net/proxy/mojo_proxy_resolver_factory.h b/net/proxy/mojo_proxy_resolver_factory.h
index 255ba093c8574f33aa5a53d8e5346f772128f149..ef924265e3bff6bf8a90c027d62a98860475d854 100644
--- a/net/proxy/mojo_proxy_resolver_factory.h
+++ b/net/proxy/mojo_proxy_resolver_factory.h
@@ -22,7 +22,7 @@ class MojoProxyResolverFactory {
   // when the connection to |req| is no longer needed.
   // Note: The connection request |req| may be resolved asynchronously.
   virtual std::unique_ptr<base::ScopedClosureRunner> CreateResolver(
-      const mojo::String& pac_script,
+      const std::string& pac_script,
       mojo::InterfaceRequest<interfaces::ProxyResolver> req,
       interfaces::ProxyResolverFactoryRequestClientPtr client) = 0;