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.
There are a few unimplemented features for this type:
1. The value setter should throw a DOMException if it is invoked on an
SVGLength that was declared readonly in another IDL file.
2. SVG::AttributeParser does not parse unit types when it parses lengths
so all SVGLength will have an "unknown" unit for now.
3. Due to (2), methods which convert between units are unimplemented.
