CSSSegmentedFontFaceCache should not know about Document.

1) Remove dependency on Document (and core/dom, for that matter)
in CSSSegmentedFontFaceCache
2) Shorten function names to make the sounds sweet.

This removes another place where CSSFontSelector is expected to know
about Document.

R=eae,eseidel,ksakamoto@chromium.org
BUG=324991

Review URL: https://codereview.chromium.org/103273002

git-svn-id: svn://svn.chromium.org/blink/trunk@163152 bbb929c8-8fbe-4397-9dbb-9b2b20218538