Fix ref counting in sideloaded font map

This code path is only used in layout tests.

R=eae@chromium.org, bungeman@chromium.org
BUG=394902

Review URL: https://codereview.chromium.org/544793002

git-svn-id: svn://svn.chromium.org/blink/trunk@181407 bbb929c8-8fbe-4397-9dbb-9b2b20218538