Handle font-weigths with 'bolder' or 'lighter' values in @font-face rules

'bolder' and 'lighter' values are valid keywords for font-weights except
if they are inside @font-face rules. The patch catches such cases.
(http://www.w3.org/TR/css3-fonts/#descdef-font-weight)

R=eae@chromium.org
BUG=410884

Review URL: https://codereview.chromium.org/542713002

git-svn-id: svn://svn.chromium.org/blink/trunk@181412 bbb929c8-8fbe-4397-9dbb-9b2b20218538