Multiple selectors setting relative font-weight names to an element result in a cumulative result

According to http://dev.w3.org/csswg/css-fonts/#font-weight-prop, the computed
weight of 'lighter' and 'bolder' are based on the inherited 'font-weight', so set
that inherited weight before calculating 'lighter' and 'bolder'.

BUG=322032

Review URL: https://codereview.chromium.org/137813004

git-svn-id: svn://svn.chromium.org/blink/trunk@166795 bbb929c8-8fbe-4397-9dbb-9b2b20218538