Rewrite the fast/css/css2-system-fonts.html test

This patch makes the fast/css/css2-system-fonts.html explicit about the
font styles expected on each platform instead of just Mac.
It also converts the test to be text only as the style information is
now displayed in text format.

This change is in preparation for a refactor of system font handling.

BUG=353932

Review URL: https://codereview.chromium.org/204113007

git-svn-id: svn://svn.chromium.org/blink/trunk@169612 bbb929c8-8fbe-4397-9dbb-9b2b20218538