[css3-background] Tests for review

Hey everyone,

I pushed 2 tests for review for the 'background-clip' property for the
'padding-box' and 'border-box' value. They are similar to
background-clip-007.html:
* contributors/google/submitted/background-clip-008.html
* contributors/google/submitted/background-clip-009.html

Any comments welcome.

Thanks,
Julien