Unprefix min-content, max-content, fit-content

This does not yet unprefix fill-available (aka fill),
because the CSSWG is not ready for that yet:
https://lists.w3.org/Archives/Public/www-style/2015Aug/0127.html

Intent to ship: https://groups.google.com/a/chromium.org/d/topic/blink-dev/2ljVa4n_ZX0/discussion

BUG=245157

Review URL: https://codereview.chromium.org/1293793007

git-svn-id: svn://svn.chromium.org/blink/trunk@200846 bbb929c8-8fbe-4397-9dbb-9b2b20218538