Add converter from css to gss.

This change is part of changes needed to introduce GSS into GWT.
This Converter is able to convert css files compatible with the
current CssResource to a css file compatible with GSS. The converter
can be used for a manual conversion or an automatic in-memory
conversion.

Change-Id: I50575f8b6864b4e0238d24d8d29ff0df5a65bebf