Web Animations API: Initialise FontBuilder when resolving keyframes

FontBuilder must be properly initialised before font properties can be
applied to a RenderStyle.

BUG=352505,352506

Review URL: https://codereview.chromium.org/203193002

git-svn-id: svn://svn.chromium.org/blink/trunk@169437 bbb929c8-8fbe-4397-9dbb-9b2b20218538