Oilpan: Move FontFaceSet to the managed heap.

R=haraken@chromium.org, sigbjornf@opera.com
BUG=

Review URL: https://codereview.chromium.org/225523003

git-svn-id: svn://svn.chromium.org/blink/trunk@171244 bbb929c8-8fbe-4397-9dbb-9b2b20218538