Rename FontDescription fields s_variant and m_italic.

s_variant is not a static field. It should be named m_variant.
(This appears to be a recent typo.)

m_italic's getter was renamed "style".
It probably makes sense for the field to follow suit.

BUG=

Review URL: https://codereview.chromium.org/223173002

git-svn-id: svn://svn.chromium.org/blink/trunk@170781 bbb929c8-8fbe-4397-9dbb-9b2b20218538