Set the scrollbarLayer as opaque after setupScrollbarLayer.

Source/web/tests/ScrollingCoordinatorChromiumTest.cpp

Index: Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
diff --git a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
index d5e412a5d29e90f67bc70e9a7cfd14a7e26885a4..b1949999804e5d540e7fcd91825fa2fb8ee893cf 100644
--- a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
+++ b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
@@ -106,6 +106,11 @@ private:
     FrameTestHelpers::WebViewHelper m_helper;
 };
 
+class GraphicsLayerForScrollTesting : public GraphicsLayer {
+public:
+    virtual WebLayer* contentsLayer() const { return GraphicsLayer::contentsLayer(); }
+};
+
 TEST_F(ScrollingCoordinatorChromiumTest, fastScrollingByDefault)
 {
     navigateTo("about:blank");
@@ -474,4 +479,28 @@ TEST_F(ScrollingCoordinatorChromiumTest, setupScrollbarLayerShouldNotCrash)
     // an empty document by javascript.
 }
 
+TEST_F(ScrollingCoordinatorChromiumTest, setupScrollbarLayerShouldSetScrollLayerOpaque)
+{
+    registerMockedHttpURLLoad("wide_document.html");
+    navigateTo(m_baseURL + "wide_document.html");
+    forceFullCompositingUpdate();
+
+    FrameView* frameView = frame()->view();
+    ASSERT_TRUE(frameView);
+
+    GraphicsLayerForScrollTesting* scrollbarGraphicsLayer = static_cast<GraphicsLayerForScrollTesting*>(frameView->layerForHorizontalScrollbar());
+    ASSERT_TRUE(scrollbarGraphicsLayer);
+
+    WebLayer* platformLayer = scrollbarGraphicsLayer->platformLayer();
+    ASSERT_TRUE(platformLayer);
+
+    WebLayer* contentsLayer = scrollbarGraphicsLayer->contentsLayer();
+    ASSERT_TRUE(contentsLayer);
+
+    // after scrollableAreaScrollbarLayerDidChange,

[enne (OOO), 2014/10/13 21:34:00]

Tiny nit: capitalize the 'A' in 'After', since it's starting a sentence.

+    // if the main frame's scrollbarLayer is opaque,
+    // contentsLayer should be opaque too.
+    ASSERT_EQ(platformLayer->opaque(), contentsLayer->opaque());
+}
+
 } // namespace