Delegated renderer: implement cc::DrawQuad::SOLID_COLOR.

I could currently only test this with --enable-overlay-scrollbars.

Background color quads still need some work in Chromium to be able to
to use this kind of quads, see
<http://code.google.com/p/chromium/issues/detail?id=114658

>.

Change-Id: I9c9bdb5c72b91827cfdb7e38e44755c6361a05cf
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>