Avoid processing anon content in nsCanvasFrame, then getting more anon content
via AccessibleCaretEventHub::Init. Instead call Init before creating the custom
content container. We could also throw a script runner at it I guess, but this
prevents the reentrancy issue.
Avoid cloning nodes during layout, just use the same node (already cloned in
InsertAnonymousContent) instead.
The RemoveChild in GetAnonymousContent to handle the reframes instead of cloning
around is a bit hacky, but I don't think it's really worth extending
PostDestroyData for this special case.
Differential Revision: https://phabricator.services.mozilla.com/D1889
