Move Alternate Family Logic into separate header

Move the alternate font family name logic out of FontCache and into a
new dedicated AlternateFontFamily.h file. This moves the non-cache
related functionality out of the FontCache simplifying the interface.

BUG=309714
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/31923005

git-svn-id: svn://svn.chromium.org/blink/trunk@160163 bbb929c8-8fbe-4397-9dbb-9b2b20218538