Don't make lots of virtual calls in shadowPseudoId

shadowPseudoId() doesn't need to be virtual anymore and it should not
call part() more than once since that's also virtual.

Review URL: https://codereview.chromium.org/35563002

git-svn-id: svn://svn.chromium.org/blink/trunk@160238 bbb929c8-8fbe-4397-9dbb-9b2b20218538