Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 459200019d647bf7c82659f05c821a8f2de435c4..3017afe560d13eed6080fcb6ebfb49212c68ced6 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1501,7 +1501,8 @@ static bool layerNeedsCompositedScrolling(PaintLayerScrollableArea::LCDTextMode
     // we should use PaintLayer::shouldPaintBackgroundOntoForeground() because we will not still get
     // LCD text unless the conditions there are met. It also unifies logic for scrolling compositing decisions.
     bool backgroundSupportsLCDText = RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled()

[flackr, 2016/08/19 17:36:53]

Can we remove the flag now?

-        && !layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha();
+        && layer->shouldPaintBackgroundOntoScrollingContentsLayer()
+        && layer->backgroundIsKnownToBeOpaqueInRect(toLayoutBox(layer->layoutObject())->paddingBoxRect());
     if (mode == PaintLayerScrollableArea::ConsiderLCDText
         && !layer->compositor()->preferCompositingToLCDTextEnabled()
         && !backgroundSupportsLCDText)