Rename FontItalics to FontStyle to match CSS terminology

Rename the FontItalics enum, the FontDescription italics/ 
setItalics methods and wrappers to FontStyle, style and style 
respectively to better match the CSS terminology. 

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

Review URL: https://codereview.chromium.org/196983014

git-svn-id: svn://svn.chromium.org/blink/trunk@169268 bbb929c8-8fbe-4397-9dbb-9b2b20218538