Remove completeURL from CSSParser's public API

There was already a FIXME to do this.  This came up while
trying to understand CSSParser's public API in:
https://codereview.chromium.org/121673002/

BUG=330389
NOTRY=true

Review URL: https://codereview.chromium.org/119113004

git-svn-id: svn://svn.chromium.org/blink/trunk@164366 bbb929c8-8fbe-4397-9dbb-9b2b20218538