Use four instead of nine bits to store font weight

Reduce the number of bytes used to store the font weight in the traits
mask from nine to four and simplify the logic for converting between
the two. Also simplify the matching logic in compareFontFaces to
compare weights instead of masks.

R=eseidel@chromium.org
TEST=Source/platform/fonts/FontDescriptionTest.cpp
BUG=

Review URL: https://codereview.chromium.org/195823004

git-svn-id: svn://svn.chromium.org/blink/trunk@169027 bbb929c8-8fbe-4397-9dbb-9b2b20218538