Document invariants in SVGUseElement::BuildPendingResource

When a <use> is scheduled to have its shadow tree built it should be
connected to the document and be the outermost <use>. The Element it
references should also be connected to the document (if it exists).

Simplify the code according to the above and add DCHECKs.

Bug: 997176
Change-Id: I4e9954df674b1dfdc5c1e674ea81df2f6e84d5da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768522Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#690391}

third_party/blink/renderer/core/svg/svg_use_element.cc

@@ -315,21 +315,23 @@ Element* SVGUseElement::ResolveTargetElement(ObserveBehavior observe_behavior) {
 }
 
 void SVGUseElement::BuildPendingResource() {
-  // Do not build the shadow/instance tree for nested <use> elements
-  // because they will get expanded in a second pass -- see
-  // ExpandUseElementsInShadowTree().
-  if (InUseShadowTree())
-    return;
+  // This runs just before computed style is updated, so this SVGUseElement
+  // should always be connected to the Document. It should also not be an
+  // SVGUseElement that is part of a shadow tree, since we should never
+  // schedule shadow tree updates for those.
+  DCHECK(!InUseShadowTree());
+  DCHECK(isConnected());
+
   DetachShadowTree();
   ClearResourceReference();
   CancelShadowTreeRecreation();
-  if (!isConnected())
-    return;
+  DCHECK(isConnected());
+
   auto* target = DynamicTo<SVGElement>(ResolveTargetElement(kAddObserver));
-  // TODO(fs): Why would the Element not be "connected" at this point?
-  if (target && target->isConnected())
+  if (target) {
+    DCHECK(target->isConnected());
     AttachShadowTree(*target);
-
+  }
   DCHECK(!needs_shadow_tree_recreation_);
 }