Most of the Shadow DOM related code are behind "dom.webcomponents.enabled" and
this pref is only used by Shadow DOM right now, so we should rename it to
"dom.webcomponents.shadowdom.enabled"
MozReview-Commit-ID: er1c7AsSSW
This is to fix the case where preference is restore to false when a testcase
ends, but nsDocument::DeleteShell is called afterwards. So, we make the
preference per-doc and set it when the document is created. The value does not
change for the lifetime of the document.
This is to fix the case where preference is restore to false when a
testcas ends, but nsDocument::DeleteShell is called afterwards. So, we
make the preference per-doc and once it is enabled for a document, it
stays enabled.
We should consider slot as a parent in ChildIterator: if slot's
`assignedNodes` is not empty, use `assignedNodes`, otherwise, use direct
children as fallback content.
Also, GetFlattenedTreeParentNodeInternal should be changed to use
`assignedSlot` instead of `DestInsertionPoints`.
Set and unset assignedNodes properly when slot is added/removed and when
slotables are added/removed/modfied. Note that assigedNodes with
flatten: true is calculated on demand.
