Mark InterfaceImpl::OnConnectionError with "override"
Mark InterfaceImpl::OnConnectionError with "override"

In Chromium, the Clang style checker has been ignoring templates. In order
to turn checking back on, I need to fix latent style issues. This is the
only one that turned up in mojo.

BUG=441916
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/808263002

collinjackson/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,chinmaygarde/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,collinjackson/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo,jianglu/mojo,collinjackson/mojo,collinjackson/mojo,afandria/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo