Global scope is always assumed to be "window"

ModelHelper.createGlobalJavaType always assumes that `window` is the correct name to use:

```
public static Type createGlobalJavaType(String packagePrefix, String globalScopeClassName) {
Type type = new Type(NAMESPACE);
type.setName(globalScopeClassName);
type.setPackageName(packagePrefix);
type.setNativeNamespace(GLOBAL_NAMESPACE);
type.addAnnotation(
builder()
.type(JS_TYPE)
.isNativeAttribute(true)
.namespaceAttribute("")
.nameAttribute("window")
.build());
return type;
}
```

However, this is only correct in a case like DomGlobal, which only makes sense in a normal window context. For classes like Global, this should instead be something like `self`, so that it works both in windows and in workers (as well as other non-browser JS contexts).

My proposal would be to change this to invoke `nameAttribute("self")`, since that seems as though it should be valid in window and worker contexts alike (see [1], [2]), but perhaps there is a better, more flexible solution?

[1] https://developer.mozilla.org/en-US/docs/Web/API/Window/self
[2] https://developer.mozilla.org/en-US/docs/Web/API/WorkerGlobalScope/self

In the mean time, objects in Global can't be used. Workarounds for others who find this:
* For `Symbol`, make your own jstype to represent it, with a static function instead of a constructor to create it
* For `JSON`, make your own `JSONType` with static methods instead of instance methods
* For NaN/Infinity/undefined and other global methods, use your own static `@JsProperty`-annotated fields.

In all of these cases, JsPackage.GLOBAL should be used as the namespace to avoid needing to reference window.