Implement findAnchor and computeRelativeOffset in ScrollAnchor.

third_party/WebKit/Source/core/layout/ScrollAnchorTest.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/layout/ScrollAnchor.h"
 
 #include "core/layout/LayoutTestHelper.h"
 
 namespace blink {
 
 static void enableScrollAnchoring(Settings& settings)
 {
     settings.setScrollAnchoringEnabled(true);
 }
 
 class ScrollAnchorTest : public RenderingTest {
     FrameSettingOverrideFunction settingOverrider() const override { return enableScrollAnchoring; }
 };
 
 TEST_F(ScrollAnchorTest, Basic)
 {
-    ScrollAnchor scrollAnchor(document().view()->layoutViewportScrollableArea());
-    scrollAnchor.clear();
-    EXPECT_EQ(nullptr, scrollAnchor.layoutObject());
+    setBodyInnerHTML(
+        "<style> body { height: 1000px } div { height: 100px } </style>"
+        "<div id='block1'>abc</div>"
+        "<div id='block2'>def</div>");
+
+    ScrollableArea* viewport = document().view()->layoutViewportScrollableArea();
+    viewport->scrollBy(DoubleSize(0, 150), UserScroll);
+    document().getElementById("block1")->setAttribute(HTMLNames::styleAttr, "height: 200px");
+    document().view()->updateAllLifecyclePhases();
+    EXPECT_EQ(250, viewport->scrollPosition().y());
 }
 
 }