Make PositionAlgorithm<Strategy>::next() to use Strategy::index()

Source/core/dom/PositionTest.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 "config.h"
 #include "core/dom/Position.h"
 
 #include "core/editing/EditingTestBase.h"
 
 namespace blink {
 
 class PositionTest : public EditingTestBase {
 };
 
+TEST_F(PositionTest, NextNodeIndex)
+{
+    const char* bodyContent = "<p id='host'>00<b id='one'>11</b><b id='two'>22</b>33</p>";
+    const char* shadowContent = "<content select=#two></content><content select=#one></content>";
+    setBodyContent(bodyContent);
+    RefPtrWillBeRawPtr<ShadowRoot> shadowRoot = setShadowContent(shadowContent);
+    Node* host = document().getElementById("host");
+    Node* two = document().getElementById("two");
+
+    EXPECT_EQ(Position(host, 3), Position(two, 2).next());
+    EXPECT_EQ(PositionInComposedTree(host, 1), PositionInComposedTree(two, 2).next());
+}
+
 TEST_F(PositionTest, NodeAsRangeLastNodeNull)
 {
     EXPECT_EQ(nullptr, Position().nodeAsRangeLastNode());
     EXPECT_EQ(nullptr, PositionInComposedTree().nodeAsRangeLastNode());
 }
 
 TEST_F(PositionTest, NodeAsRangeLastNode)
 {
     const char* bodyContent = "<p id='p1'>11</p><p id='p2'></p><p id='p3'>33</p>";
     setBodyContent(bodyContent);
@@ skipping 102 matching lines @@
     const char* bodyContent = "<p id='host'>00<b id='one'>11</b>22</p>";
     const char* shadowContent = "";
     setBodyContent(bodyContent);
     RefPtrWillBeRawPtr<ShadowRoot> shadowRoot = setShadowContent(shadowContent);
     RefPtrWillBeRawPtr<Element> host = document().getElementById("host");
 
     EXPECT_EQ(PositionInComposedTree(host, PositionAnchorType::AfterChildren), toPositionInComposedTree(positionInDOMTree(*shadowRoot, 0)));
 }
 
 } // namespace blink