Tighten up FontFallbackList's relatonship with Font.

FontFallbackList is effectively owned by Font, so it's a little weird
that Font queries its fallback list using itself. In fact, it actually
means FontDescription all the time. We should fix that.

Also -- move getFontData from FontCache to FontFallbackList. It is
not a good final resting place, but better than the extra plumbing
we had before.

R=eae
BUG=315783

Review URL: https://codereview.chromium.org/63173007

git-svn-id: svn://svn.chromium.org/blink/trunk@161579 bbb929c8-8fbe-4397-9dbb-9b2b20218538