Generate scroll/clip display item hierarchy for SPv2

Source/platform/graphics/paint/DisplayItem.h

 // Copyright 2014 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.
 
 #ifndef DisplayItem_h
 #define DisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/graphics/ContiguousContainer.h"
 #include "platform/graphics/paint/DisplayItemClient.h"
@@ skipping 94 matching lines @@
         CachedDrawingFirst,
         CachedDrawingLast = CachedDrawingFirst + DrawingLast - DrawingFirst,
 
         ClipFirst,
         ClipBoxPaintPhaseFirst = ClipFirst,
         ClipBoxPaintPhaseLast = ClipBoxPaintPhaseFirst + PaintPhaseMax,
         ClipColumnBoundsPaintPhaseFirst,
         ClipColumnBoundsPaintPhaseLast = ClipColumnBoundsPaintPhaseFirst + PaintPhaseMax,
         ClipLayerFragmentPaintPhaseFirst,
         ClipLayerFragmentPaintPhaseLast = ClipLayerFragmentPaintPhaseFirst + PaintPhaseMax,
+        ClipDescendantStackingContext,
         ClipFileUploadControlRect,
         ClipFrameToVisibleContentRect,
         ClipFrameScrollbars,
         ClipLayerBackground,
         ClipLayerColumnBounds,
         ClipLayerFilter,
         ClipLayerForeground,
         ClipLayerParent,
         ClipLayerOverflowControls,
         ClipNodeImage,
@@ skipping 29 matching lines @@
         BeginFilter,
         EndFilter,
         BeginCompositing,
         EndCompositing,
         BeginTransform,
         EndTransform,
         BeginClipPath,
         EndClipPath,
         BeginFixedPosition,
         EndFixedPosition,
-        BeginFixedPositionContainer,
-        EndFixedPositionContainer,
+        FixedPositionContainer,

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

Won't we still need begin/end pairs for multiple fixed-pos containers (nested,
or not). For example: http://jsfiddle.net/progers/euhb41Lf/1

Can we pull the fixed pos logic into a separate patch, maybe after the big one?
I think the direction looks good overall, just want to simplify this patch.

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

Previously when I added it I was thinking the fixed-pos container will establish
a scope, and fixed-pos display item will refer to the innermost enclosing one.

Now I think the anchor/warp is a simpler approach (also more flexible). We may
rename them to [Anchor|Warp]DisplayItem in the future if we also want to
implement de-duping for O(n^2) scroll display item problem.
Sure we can! Just that fixed-pos will be broken in the meantime.

 
         BeginSubsequence,
         EndSubsequence,
         CachedSubsequence,
 
         UninitializedType,
         TypeLast = UninitializedType
     };
 
     static_assert(TableCollapsedBorderBase >= TableCollapsedBorderUnalignedBase, "TableCollapsedBorder types overlap with other types");
@@ skipping 201 matching lines @@
     bool isEndAndPairedWith(DisplayItem::Type otherType) const override = 0;
 #endif
 
 private:
     bool isEnd() const final { return true; }
 };
 
 } // namespace blink
 
 #endif // DisplayItem_h