Rename FontSmallCaps to FontVariant to match CSS terminology

Rename the FontSmallCaps enum, the FontDescription smallCaps/
setSmallCaps methods and wrappers to FontVariant, variant and setVariant
respectively to better match the CSS terminology.

R=dglazkov@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/197753012

git-svn-id: svn://svn.chromium.org/blink/trunk@169258 bbb929c8-8fbe-4397-9dbb-9b2b20218538