Bug 1450250: Make <animate> target the light DOM parent, not the flattened tree parent. r=heycam

This matches Chrome:

  https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/svg/animation/svg_smil_element.cc?l=260&rcl=325eafc631f396944801f17675a7092b216d60ca

And we rely on that to animate <use> with SMIL, like:

<use href="#foo">
  <animate ...>
</use>

There, the flattened tree parent of the <animate> element is null, because <use>
is a shadow root without slots.

Similarly we need to skip the GetCtx check which seems unneeded, and also is not
correct since <animate> can animate when it's out of the flattened tree.

Differential Revision: https://phabricator.services.mozilla.com/D2194

MozReview-Commit-ID: 13hRCOj500n

dom/svg/SVGAnimationElement.cpp

@@ -72,7 +72,10 @@ SVGAnimationElement::GetTargetElementContent()
              "if we don't have an xlink:href or href attribute");
 
   // No "href" or "xlink:href" attribute --> I should target my parent.
-  return GetFlattenedTreeParentElement();
+  //
+  // Note that we want to use GetParentElement instead of the flattened tree to
+  // allow <use><animate>, for example.
+  return GetParentElement();
 }
 
 bool
@@ -168,15 +171,6 @@ SVGAnimationElement::BindToTree(nsIDocument* aDocument,
                                                     aCompileEventHandlers);
   NS_ENSURE_SUCCESS(rv,rv);
 
-  // XXXdholbert is GetCtx (as a check for SVG parent) still needed here?
-  if (!GetCtx()) {
-    // No use proceeding. We don't have an SVG parent (yet) so we won't be able
-    // to register ourselves etc. Maybe next time we'll have more luck.
-    // (This sort of situation will arise a lot when trees are being constructed
-    // piece by piece via script)
-    return NS_OK;
-  }
-
   // Add myself to the animation controller's master set of animation elements.
   if (nsIDocument* doc = GetComposedDoc()) {
     nsSMILAnimationController* controller = doc->GetAnimationController();
@@ -208,7 +202,7 @@ SVGAnimationElement::BindToTree(nsIDocument* aDocument,
 void
 SVGAnimationElement::UnbindFromTree(bool aDeep, bool aNullParent)
 {
-  nsSMILAnimationController *controller = OwnerDoc()->GetAnimationController();
+  nsSMILAnimationController* controller = OwnerDoc()->GetAnimationController();
   if (controller) {
     controller->UnregisterAnimationElement(this);
   }