C++ bindings: support using a callback as connection error handler.

mojo/public/cpp/bindings/lib/connector.cc

Index: mojo/public/cpp/bindings/lib/connector.cc
diff --git a/mojo/public/cpp/bindings/lib/connector.cc b/mojo/public/cpp/bindings/lib/connector.cc
index 89499dccce9d11f8ad31a05b7d341f6db1253404..24d993d12865f4fb5b33fba537903b20daeba34a 100644
--- a/mojo/public/cpp/bindings/lib/connector.cc
+++ b/mojo/public/cpp/bindings/lib/connector.cc
@@ -14,8 +14,7 @@ namespace internal {
 
 Connector::Connector(ScopedMessagePipeHandle message_pipe,
                      const MojoAsyncWaiter* waiter)
-    : error_handler_(nullptr),
-      waiter_(waiter),
+    : waiter_(waiter),
       message_pipe_(message_pipe.Pass()),
       incoming_receiver_(nullptr),
       async_wait_id_(0),
@@ -198,8 +197,7 @@ void Connector::CancelWait() {
 void Connector::NotifyError() {
   error_ = true;
   CloseMessagePipe();
-  if (error_handler_)
-    error_handler_->OnConnectionError();
+  connection_error_handler_.Run();
 }
 
 }  // namespace internal