Unlike ensure_web_prototype<T>(), the cached version doesn't require the
prototype type to be fully formed, so we can use it without including
the FooPrototype.h header. It's also a bit less verbose. :^)
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.
