Bug 874950. Record on display items if they are inside a fixed position subtree. r=roc
Bug 874950. Don't let fixed position conclude occlude async scrolled content. r=roc

It would be incorrect to occlude async scrolled content.

Instead of doing a lot of extra frame tree walking just use the flags on display items to tell if they are in fixed position subtrees.

Also, don't let the fixed position flag on display items get set if we start building display items inside a subdocument. If didn't do this non-fixed content in a subdocument that is in fixed content in the parent document would be marked as in fixed position, and no occlusion would occur at all. This means that the fix applies to slightly less cases which are less important so that a more important case works the same as before.