Followup to CSSTokenizer memory usage optimization

Make sure to measure the same input string as we're going to parse.

BUG=358667,352544

Review URL: https://codereview.chromium.org/220113006

git-svn-id: svn://svn.chromium.org/blink/trunk@170636 bbb929c8-8fbe-4397-9dbb-9b2b20218538