Removed unused scrollbar pseudo classes.

::-webkit-scrollbar-back and ::-webkit-scrollbar-forward had PseudoType
constants, but the strings were not recognized and no other code for
handling them present.

Review URL: https://codereview.chromium.org/659833002

git-svn-id: svn://svn.chromium.org/blink/trunk@183795 bbb929c8-8fbe-4397-9dbb-9b2b20218538