Make FontFace ActiveDOMObject

This prevents V8 wrapper of FontFace objects from getting garbage
collected while loading. This is necessary because the "loaded" attribute
uses ScriptPromiseProperty and it needs the FontFace wrapper to be alive
to deliver the Promise resolution/rejection.

BUG=413901
TEST=fast/css/fontface-load-promise-after-gc.html

Review URL: https://codereview.chromium.org/574643002

git-svn-id: svn://svn.chromium.org/blink/trunk@182151 bbb929c8-8fbe-4397-9dbb-9b2b20218538