Use Traversal<>::firstAncestor() API more in the code base

Source/core/dom/shadow/InsertionPoint.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 117 matching lines @@
 
 bool InsertionPoint::shouldUseFallbackElements() const
 {
     return isActive() && !hasDistribution();
 }
 
 bool InsertionPoint::canBeActive() const
 {
     if (!isInShadowTree())
         return false;
-    for (Node* node = parentNode(); node; node = node->parentNode()) {
-        if (node->isInsertionPoint())
-            return false;
-    }
-    return true;
+    return !Traversal<InsertionPoint>::firstAncestor(*this);
 }
 
 bool InsertionPoint::isActive() const
 {
     if (!canBeActive())
         return false;
     ShadowRoot* shadowRoot = containingShadowRoot();
     if (!shadowRoot)
         return false;
     if (!isHTMLShadowElement(*this) || shadowRoot->descendantShadowElementCount() <= 1)
@@ skipping 133 matching lines @@
         if (!insertionPoints)
             return;
         for (size_t i = 0; i < insertionPoints->size(); ++i)
             results.append(insertionPoints->at(i).get());
         ASSERT(current != insertionPoints->last().get());
         current = insertionPoints->last().get();
     }
 }
 
 } // namespace blink