Remove useless checks for webkit-min/max

This patch removes a couple of checks for webkit-min/max, which were
accidentally left in when removing parsing support:
https://src.chromium.org/viewvc/blink?revision=171550&view=revision

Review URL: https://codereview.chromium.org/350573003

git-svn-id: svn://svn.chromium.org/blink/trunk@177199 bbb929c8-8fbe-4397-9dbb-9b2b20218538