Generate scroll/clip display item hierarchy for SPv2

Source/core/paint/LayerClipRecorder.h

Index: Source/core/paint/LayerClipRecorder.h
diff --git a/Source/core/paint/LayerClipRecorder.h b/Source/core/paint/LayerClipRecorder.h
index b1f93ed8a892d6c249456fd98c1990c68323b069..8534e014334e2851ecb50491b2c7a633a7556404 100644
--- a/Source/core/paint/LayerClipRecorder.h
+++ b/Source/core/paint/LayerClipRecorder.h
@@ -16,6 +16,7 @@ namespace blink {
 
 class ClipRect;
 class GraphicsContext;
+class LayoutBox;
 class LayoutBoxModelObject;
 
 class CORE_EXPORT LayerClipRecorder {
@@ -35,7 +36,14 @@ public:
     // same time we pass a fragmentOffset, so that we can translate from flow thread coordinates to
     // visual coordinates. This may look rather confusing/redundant, but it is needed for rounded
     // border clipping. Would be nice to clean up this.
-    explicit LayerClipRecorder(GraphicsContext&, const LayoutBoxModelObject&, DisplayItem::Type, const ClipRect&, const DeprecatedPaintLayerPaintingInfo* localPaintingInfo, const LayoutPoint& fragmentOffset, PaintLayerFlags, BorderRadiusClippingRule = IncludeSelfForBorderRadius);
+    LayerClipRecorder(GraphicsContext&, const LayoutBoxModelObject&, DisplayItem::Type, const ClipRect&, const DeprecatedPaintLayerPaintingInfo*, const LayoutPoint& fragmentOffset, PaintLayerFlags, BorderRadiusClippingRule = IncludeSelfForBorderRadius);
+
+    // Note: This constructor is designed for SPv2, and has drastically different semantics than
+    // the other version above. In the old layer painting model, a layer is responsible for
+    // calculating the accumulated clip from ancestors, and applies the clip to itself.
+    // In SPv2, it is the layer that establishes overflow clipping who is responsible to clip
+    // its descendants. This constructor takes a layer, and only issues clip for that layer.
+    LayerClipRecorder(GraphicsContext&, const LayoutBox&, const LayoutPoint& paintOffset);

[pdr., 2015/09/03 06:16:59]

Because the two modes for LayerClipRecorder are so different, lets add a second
class in this file called LayerClipRecorderForSlimmingPaintV2 instead of
re-using the existing class. The comment is very helpful so lets keep it, just
tweaked for a new class.

[trchen, 2015/09/04 06:10:15]

Done.

 
     ~LayerClipRecorder();