io.open() will fail on Mac when no encoding is passed as parameter.
It will make the build of chromium fail.
This patch will set the encoding to utf-8.

BUG=271567

Review URL: https://chromiumcodereview.appspot.com/22812004

git-svn-id: svn://svn.chromium.org/blink/trunk@156250 bbb929c8-8fbe-4397-9dbb-9b2b20218538