Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.h

 // Copyright 2016 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/frame/FrameView.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/layout/api/LayoutViewItem.h"
 #include "core/paint/PaintLayer.h"
 #include "core/paint/PaintLayerScrollableArea.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace blink {
 
 class PaintLayerScrollableAreaTest : public RenderingTest {
 public:
     PaintLayerScrollableAreaTest()
         : RenderingTest(SingleChildFrameLoaderClient::create())
     { }
 
+    bool shouldPaintBackgroundOntoScrollingContentsLayer(const char* elementId)
+    {
+        PaintLayer* paintLayer = toLayoutBlock(getLayoutObjectByElementId(elementId))->layer();
+        return paintLayer->shouldPaintBackgroundOntoScrollingContentsLayer();
+    }
+
 private:
     void SetUp() override
     {
         RenderingTest::SetUp();
         enableCompositing();
     }
 
     void TearDown() override
     {
         RenderingTest::TearDown();
     }
 };
 
 }