`FlattenedChildIterator` is a non-template class and work only for the
flattened tree (i.e., in the `TreeKind::Flat` mode).
To centralize the implementation to access children from a parent node
in the various kinds of tree except `ShadowIncludingDOM`, this patch
makes it a template class and support the simple DOM (for
`TreeKind::DOM`), the flattened tree (for`TreeKind::Flat`) and also the
flattened tree for selection whose `TreeKind` will be added by the next
patch.
The reason why this patch does not support
`TreeKind::ShadowIncludingDOM` is, it is not clear that which children
should be handled when the parent is a shadow host, i.e., whether the
children of the host or the `ShadowRoot`. Therefore, the caller should
use `ShadowRoot` if the users want to list up the the children from a
host. Then, the code is really clear about what it does.
Differential Revision: https://phabricator.services.mozilla.com/D281303
Masayuki Nakano
dc6c60d7f2