Remove unused -webkit-min() and -webkit-max()

-webkit-min() and -webkit-max() are not in any specs and only partially
implemented (we only appear to have parsing logic for them). The min()
and max() functions were previously slated for addition in CSS3 Values
and Units, but were removed due to the complexity added[1][2].
This may be introducted in CSS4 Values and Units[3], but for now I don't
see the point of keeping this in the codebase.

[1] http://lists.w3.org/Archives/Public/www-style/2010Sep/0233.html
[2] http://lists.w3.org/Archives/Public/www-style/2011Oct/0735.html
[3] http://www.w3.org/Style/CSS/Tracker/products/33

Review URL: https://codereview.chromium.org/238683003

git-svn-id: svn://svn.chromium.org/blink/trunk@171550 bbb929c8-8fbe-4397-9dbb-9b2b20218538