Web Animations API: Ensure font animations update style correctly

Animating properties like font-style via element.animate() cause the font
style to be dirtied, we must ensure the font attached to the RenderStyle
gets updated for these cases.

BUG=372687

Review URL: https://codereview.chromium.org/296703002

git-svn-id: svn://svn.chromium.org/blink/trunk@174353 bbb929c8-8fbe-4397-9dbb-9b2b20218538