Add nbarth@ as "per-file *.idl" owner

I'm v. familiar with IDL issues, so I'm a good reviewer for .idl changes;
this should help get me suggested in Rietveld and reduce load on other
reviewers.

TBR=eseidel

Review URL: https://codereview.chromium.org/197073008

git-svn-id: svn://svn.chromium.org/blink/trunk@169193 bbb929c8-8fbe-4397-9dbb-9b2b20218538