Make style building for 'font-family' less custom.

This patch changes style building for 'font-family', such that the
CSSValue -> FontFamily conversion happens on the outside, rather than
passing the CSSValue directly to the FontBuilder. This aligns building
of 'font-family' more with how everything else works, and also removes the
final dependency on CSSValue in FontBuilder.

The in-flight struct FontDescription::FamilyDescription is used to
transfer FontFamily+GenericFamilyType from StyleBuilder::applyProperty to
FontBuilder.

Note: The unrelated FIXME was intentionally removed. I don't consider it
valid (at least not anymore).

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/602373003

git-svn-id: svn://svn.chromium.org/blink/trunk@182839 bbb929c8-8fbe-4397-9dbb-9b2b20218538