Revert &quotCSS: Drop the quirks-mode exception for CSS MIME types.&quot

      This is a speculative revert to see if it unblocks the Blnk roll;
we're seeing various failures in PPAPI input event tests and
sheriff-o-matic's build triangulation is pointing at this rev.

TBR=mkwst@chromium.org
BUG=419383

Review URL: https://codereview.chromium.org/904143002

git-svn-id: svn://svn.chromium.org/blink/trunk@189711 bbb929c8-8fbe-4397-9dbb-9b2b20218538