Remove MediaValues' dependency on style

MediaValues needed a RenderStyle for default font calculations. However, since a renderer is not always available,
and the default font size can also be concluded from the LocalFrame, this dependency can be removed.

BUG=357586

Review URL: https://codereview.chromium.org/242883002

git-svn-id: svn://svn.chromium.org/blink/trunk@171985 bbb929c8-8fbe-4397-9dbb-9b2b20218538