This way we reuse the same machinery everywhere for the content property.
The only difference is that we need to look at the parent style for content
instead of just our style, and at a given index.
Again, this is fine because changing content reframes, so no chance to mess up.
This allows the generated content stuff to not implement nsImageLoadingContent
and all that stuff, nor deal with events, which makes it much simpler IMO.
Now it just tracks an index. We may not even need for it to be an HTML element,
but I've kept that for now.
I added a crashtest that used to crash because of the bogus
nsCSSFrameConstructor code which trusted the node name without checking it was
native anonymous.
Differential Revision: https://phabricator.services.mozilla.com/D1897
MozReview-Commit-ID: 1pAzIvRRVnL