Match spec for font-weight: bolder|lighter

Currently, bolder and lighter change font-weight in a non-compliant way
The spec (http://www.w3.org/TR/css3-fonts/#bolderlighter) defines
exactly what the computed values should be given the inherited weight
values, so we should match those.

The removed FIXME's for selecting the next lightest/heaviest weight from
the used font family seems to refer to behaviour from
CSS1 (http://www.w3.org/TR/CSS1/#font-weight), while this is replaced
with a simpler procedure for resolving bolder and lighter in font weights
in CSS2 (http://www.w3.org/TR/CSS2/fonts.html#propdef-font-weight) and
CSS3 (link above).

BUG=242041

Review URL: https://chromiumcodereview.appspot.com/15994009

git-svn-id: svn://svn.chromium.org/blink/trunk@153116 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed