[Bindings] When generating a union of (callback, dict) we cannot convert from pointer-to-derived to pointer-to-base.

[Ryan Reno]

IDL like this:

typedef (MyCallback, MyDict) MyUnion; 
interface MyInterface {
    undefined myFunction(optional MyUnion = {})
}

Will generate a function body for myFunction which will have as a conversion result for the argument an optional<variant<RefPtr<JSMyCallback>, MyDict>>. Which means that we *must* use a RefPtr to the derived JSWrapper class in WebCore rather than the base Wrapped class. This is because C++ won't let us convert from variant<Derived*, ...> to variant<Base*, ...> as they are distinct types.

There must be some way to either coax the C++ compiler to do what we mean or to modify the code generator to output the variant<Base*, ...> type as the result of the conversion.

[Radar WebKit Bug Importer]

<rdar://problem/131851145>