[HTML] Remove the dependency on HTMLElementImp::eval() #3

(HTMLElementImp::handleMutationBackground): New
(HTMLElementImp::evalBackground): Remove; all callers changed.
[HTML] Remove the dependency on HTMLElementImp::eval() #4

(HTMLElementImp::handleMutationColor): New
(HTMLElementImp::evalColor, HTMLElementImp::evalBgcolor): Remove; all callers changed.