Add support for font-weight: lighter for web fonts

Update FontFace::traitsMask to add the missing CSSValueLighter keyword
thereby supporting font-weight: lighter. Also add ASSERT to ensure that
all supported weights are handled.

TEST=fast/css/webfont-lighter-weight-crash.html
BUG=334684
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/130783004

git-svn-id: svn://svn.chromium.org/blink/trunk@165245 bbb929c8-8fbe-4397-9dbb-9b2b20218538