Simplify logic for parseFontFaceUnicodeRange

This patch simplifies the logic in parseFontFaceUnicodeRange, removing
the two locals 'failed' and 'operatorExpected'. If we have trailing
values then we still return a CSSValue but parseValue will ignore it.

BUG=404023

Review URL: https://codereview.chromium.org/510273002

git-svn-id: svn://svn.chromium.org/blink/trunk@181146 bbb929c8-8fbe-4397-9dbb-9b2b20218538