Previously we were using the HTML parse_dimension_value method for the
height and width attributes of an SVG element. These attributes should
however be treated as css properties instead and thus also support
calc() and absolute units so we use the css parser for this instead.
Note that as of this commit, there aren't any such throwers, and the
call site in Heap::allocate will drop exceptions on the floor. This
commit only serves to change the declaration of the overrides, make sure
they return an empty value, and to propagate OOM errors frm their base
initialize invocations.
This needs to happen before prototype/constructor intitialization can be
made lazy. Otherwise, GC could run during the C++ constructor and try to
collect the object currently being created.
