Don't prefetch images when they exist within <noscript> tags

[burg]

The hubbub-based parser will eagerly tell the ImageCacheTask to prefetch images for any tags of the form <img src="...">. That shouldn't happen if the element is a descendant of a <noscript> tag, since Servo does not have a noscript option.

[bzbarsky]

Specifically, we need to watch out for cases like https://bugzilla.mozilla.org/show_bug.cgi?id=642908

[jdm]

This should be pretty easy to accomplish using the ancestors() iterator on Node.

[bzbarsky]

At the point when the prefetch runs you don't have a Node, right? You just have the parser's tokenization state.

[jdm]

No, right now prefetching is controlled by HTMLImageElement::AfterSetAttr.

[bzbarsky]

That's not prefetching, then. That's just loading the image....

[guillaumebort]

I wanted to give a try to this one, but it seems that it is already not an issue anymore.

The pull request is here #3934

I have added a test cases to document.images, but I'm not totally sure it tests the underlying image cache. Let me know if there is another better way to test that.

(note: I've also manually tested that no Prefetch messages are sent in case of an <img> tag included in <noscript>)