Check V0 insertion point before shadow root.

It is possible to create a shadow root on a Shadow DOM V0 insertion
point. However, if it's an active insertion point, we render the
distributed children and not the shadow subtree. Make sure we traverse
the distributed children in that case, and not the shadow tree, when
FlatTreeStyleRecalc is enabled.

TEST=fast/dom/shadow/shadowroot-of-insertionpoint.html

Bug: 972752
Change-Id: I9158a5d669764a049acb9780dccc4ae1b0ff87b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1875735
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708971}

third_party/blink/renderer/core/dom/element.cc

@@ -3010,7 +3010,10 @@ void Element::RecalcStyle(const StyleRecalcChange change) {
 
   if (child_change.TraverseChildren(*this)) {
     SelectorFilterParentScope filter_scope(*this);
-    if (ShadowRoot* root = GetShadowRoot()) {
+    if (IsActiveV0InsertionPoint(*this)) {
+      To<V0InsertionPoint>(this)->RecalcStyleForInsertionPointChildren(
+          child_change);
+    } else if (ShadowRoot* root = GetShadowRoot()) {
       if (RuntimeEnabledFeatures::FlatTreeStyleRecalcEnabled()) {
         root->RecalcDescendantStyles(child_change);
       } else {
@@ -3020,9 +3023,6 @@ void Element::RecalcStyle(const StyleRecalcChange change) {
       }
     } else if (auto* slot = ToHTMLSlotElementIfSupportsAssignmentOrNull(this)) {
       slot->RecalcStyleForSlotChildren(child_change);
-    } else if (IsActiveV0InsertionPoint(*this)) {
-      To<V0InsertionPoint>(this)->RecalcStyleForInsertionPointChildren(
-          child_change);
     } else {
       RecalcDescendantStyles(child_change);
     }