Make slots in non-shadow trees participate in a flat tree

third_party/WebKit/Source/core/dom/shadow/FlatTreeTraversalTest.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/shadow/FlatTreeTraversal.h"
 
 #include "bindings/core/v8/ExceptionStatePlaceholder.h"
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/dom/Node.h"
@@ skipping 565 matching lines @@
       "<div id='parent'>"
       "<slot>"
       "<div id='child1'></div>"
       "<div id='child2'></div>"
       "</slot>"
       "</div>";
 
   setupDocumentTree(mainHTML);
   Element* body = document().body();
   Element* parent = body->querySelector("#parent");
+  Element* slot = body->querySelector("slot");
   Element* child1 = body->querySelector("#child1");
   Element* child2 = body->querySelector("#child2");
 
-  EXPECT_EQ(child1, FlatTreeTraversal::firstChild(*parent));
+  EXPECT_EQ(slot, FlatTreeTraversal::firstChild(*parent));
+  EXPECT_EQ(child1, FlatTreeTraversal::firstChild(*slot));
   EXPECT_EQ(child2, FlatTreeTraversal::nextSibling(*child1));
   EXPECT_EQ(nullptr, FlatTreeTraversal::nextSibling(*child2));
-  EXPECT_EQ(parent, FlatTreeTraversal::parent(*child1));
-  EXPECT_EQ(parent, FlatTreeTraversal::parent(*child2));
+  EXPECT_EQ(slot, FlatTreeTraversal::parent(*child1));
+  EXPECT_EQ(slot, FlatTreeTraversal::parent(*child2));
+  EXPECT_EQ(parent, FlatTreeTraversal::parent(*slot));
 }
 
 }  // namespace blink