Add myself to libwebp OWNERS list.

It would be helpful for the WebP team, now that I'm a committer.

BUG=
R=fbarchard@chromium.org

Review URL: https://codereview.chromium.org/75773004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235977 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235977