Revert the feature of the <shadow> element as 'a function call' to the previous behavior.

Source/core/dom/shadow/InsertionPoint.cpp

Index: Source/core/dom/shadow/InsertionPoint.cpp
diff --git a/Source/core/dom/shadow/InsertionPoint.cpp b/Source/core/dom/shadow/InsertionPoint.cpp
index 4ac11b49488c9eb844816f0d92648b9680be7864..8cc0fb45dc3e2d630b280c84a0241d3c5cc8c9bc 100644
--- a/Source/core/dom/shadow/InsertionPoint.cpp
+++ b/Source/core/dom/shadow/InsertionPoint.cpp
@@ -134,16 +134,9 @@ bool InsertionPoint::canBeActive() const
 {
     if (!isInShadowTree())
         return false;
-    bool foundShadowElementInAncestors = false;
-    bool thisIsContentHTMLElement = hasTagName(contentTag);
     for (Node* node = parentNode(); node; node = node->parentNode()) {
-        if (node->isInsertionPoint()) {
-            // For HTMLContentElement, at most one HTMLShadowElement may appear in its ancestors.
-            if (thisIsContentHTMLElement && node->hasTagName(shadowTag) && !foundShadowElementInAncestors)
-                foundShadowElementInAncestors = true;
-            else
-                return false;
-        }
+        if (node->isInsertionPoint())
+            return false;
     }
     return true;
 }