Send fatal proxy errors to the network delegate.

net/http/http_network_delegate.h

Index: net/http/http_network_delegate.h
diff --git a/net/http/http_network_delegate.h b/net/http/http_network_delegate.h
index 1533bc3711e818b2db4b68f52dce113b2ec6a9a2..b495f919eefc719a4c1bc022626fa1ef0b415f04 100644
--- a/net/http/http_network_delegate.h
+++ b/net/http/http_network_delegate.h
@@ -20,6 +20,9 @@ class HttpNetworkDelegate {
   // read/write |headers| before they get sent out.
   virtual void OnSendHttpRequest(HttpRequestHeaders* headers) = 0;
 
+  // Called right after the response started.
+  virtual void OnResponseStarted(net::URLRequest* request) = 0;
+
  protected:
   virtual ~HttpNetworkDelegate() {}
 };