When calling constructors, retrieve the global object from the callee rather than the JSContext.
Return a JS<Window> from global_object_for_{dom,js}_object.
auto merge of #1809 : Ms2ger/servo/constructor-global-from-object, r=jdm

...rather than the JSContext.

Requires <https://github.com/mozilla-servo/rust-mozjs/pull/51>.