This patch has the side-effect of changing the representation of '0'
values in properties added by SVG to use the number representation
rather than the length representation. This requires marking the
stroke-dashoffset:0 test in test_value_computation.html as known to
fail, since computed style code now returns 0 rather than 0px, and also
requires changing some inputs and results in some SMIL tests (which
seems like it's showing a bug that we can't interpolate between
equivalent values when we ought to be able to do so).
