Rename CSSSegmentedFontFaceCache to FontFaceCache.

This class uses FontFace objects as key, so "FontFaceCache" describes it better.

BUG=

Review URL: https://codereview.chromium.org/148813003

git-svn-id: svn://svn.chromium.org/blink/trunk@166004 bbb929c8-8fbe-4397-9dbb-9b2b20218538