Differentiate between associated windows and real document views

[jdm]

#1072 is getting rid of optional windows, since we require them in order to access the JSContext everywhere. However, a newly parsed Document obtained via DOMParser should have a null defaultView, so we need a way to reflect that.

[Ms2ger]

@bholley probably has thoughts on how we deal with JSContexts. As for the scope, we might want a more general global object rather than requiring a Window (so we don't have to rewrite everything for workers).

[bholley]

The JSContext should live on the runtime. JSContexts are going away soon in SM. Conceptually, they should be 1-to-1 with JSRuntimes in servo.

[jdm]

We've got code right now that sets the private field of a JSContext to a pointer to the associated Page right now, so that will need some consideration in the presence of iframes and such.

[bholley]

OMGNO! We should not be doing that. I've been working on getting rid of that nonsense from Gecko for over a year now.

How easy is it to rip out? What do we need it for?

[jdm]

http://mxr.mozilla.org/servo/search?string=page_from_context
In the codegen, the Page is used to store references to proxy handlers and other necessary data, and we grab the Window reference via it. Everywhere else, it looks like we use it to obtain the window object for GetParentObject. As I remarked on IRC, we can probably do something like JS_GetGlobalForObject on a given reflector and unwrap that to Window, giving us our global reference as well as a reference to the Page as well.

[jdm]

The great JSContext scare is behind us, and they're 1:1 with runtimes.

[jdm]

Related: #4806

[Ms2ger]

Fixed in #9603.