Mark parsing routines with ENABLE(SVG_FONTS)

Marking these SVG Fonts specific parsing routines makes it
harder to forget to remove them at the time SVG Fonts are removed.

BUG=242735

Review URL: https://codereview.chromium.org/540473002

git-svn-id: svn://svn.chromium.org/blink/trunk@181340 bbb929c8-8fbe-4397-9dbb-9b2b20218538