Reduce code duplication for parsing clip-path/filter

The parsing logic for clip-path and filter is identical to that for
marker-start, etc.

BUG=404023

Review URL: https://codereview.chromium.org/475303002

git-svn-id: svn://svn.chromium.org/blink/trunk@180346 bbb929c8-8fbe-4397-9dbb-9b2b20218538