Move word spacing and letter spacing concepts out of Font.

Both of these things belong in FontDescription.

BUG=319475
R=ksakamoto@chromium.org, eae

Review URL: https://codereview.chromium.org/153753003

git-svn-id: svn://svn.chromium.org/blink/trunk@166690 bbb929c8-8fbe-4397-9dbb-9b2b20218538