Update the WindowTimers interface to the current specification.
Remove support for returning values through outparams.
auto merge of #1890 : Ms2ger/servo/Window-idl, r=jdm
auto merge of #1895 : Ms2ger/servo/outparams, r=jdm
script: Fix background color of Acid2.

There were two problems here: (1) we did not process style sheets with an
unexpected `rel` attribute but a correct MIME type; (2) we did not
consider `none` a valid value for the `background` property.