Make StyleResolver a FontSelectorClient.

Address a 2-year old comment that made sense. Instead of poking
at Document and StyleResolver directly from CSSStyleResolver,
turn StyleResolver into a FontSelectorClient.

BUG=291287

Review URL: https://codereview.chromium.org/25507005

git-svn-id: svn://svn.chromium.org/blink/trunk@158794 bbb929c8-8fbe-4397-9dbb-9b2b20218538