Fix SVG crash for v0 distribution into foreignObject.

We require a parent element to be an SVG element for non-svg-root
elements in order to create a LayoutObject for them. However, we checked
the light tree parent element, not the flat tree one which is the parent
for the layout tree construction. Note that this is just an issue in
Shadow DOM v0 since v1 does not allow shadow roots on SVG elements.

Bug: 915469
Change-Id: Id81843abad08814fae747b5bc81c09666583f130
Reviewed-on: https://chromium-review.googlesource.com/c/1382494
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617487}
void SVGElement::ChildrenChanged(const ChildrenChange& change) {
  Element::ChildrenChanged(change);

  InvalidateInstances();
}

void SVGElement::ChildrenChanged(const ChildrenChange& change) {
  Element::ChildrenChanged(change);

  InvalidateInstances();
}
