Move reading font family names from settings to CSSFontSelector.

CSSSegmentedFontFaceCache should not know about FontData or
Settings (or Frame for that matter), and the logic for
getting FontData for generic fonts should not live there.

I also massaged the actual logic into a slightly nicer shape.

R=eae,ksakamoto@chromium.org,eseidel
BUG=315783

Review URL: https://codereview.chromium.org/81543002

git-svn-id: svn://svn.chromium.org/blink/trunk@162493 bbb929c8-8fbe-4397-9dbb-9b2b20218538