Implement SVG masking with slimming paint

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/paint/DisplayItemClient.h"
 #include "wtf/Assertions.h"
@@ skipping 50 matching lines @@
         DebugRedFill,
         DragImage,
         LinkHighlight,
         PageOverlay,
         PageWidgetDelegateBackgroundFallback,
         PopupContainerBorder,
         PopupListBoxBackground,
         PopupListBoxRow,
         Resizer,
         SVGFilter,
+        SVGMask,
         ScrollbarCorner,
         ScrollbarHorizontal,
         ScrollbarTickMark,
         ScrollbarVertical,
         SelectionGap,
         VideoBitmap,
         ViewBackground,
         DrawingLast = ViewBackground,
 
         CachedFirst,
@@ skipping 203 matching lines @@
     virtual bool isEndAndPairedWith(const DisplayItem& other) const override = 0;
 #endif
 
 private:
     virtual bool isEnd() const override final { return true; }
 };
 
 } // namespace blink
 
 #endif // DisplayItem_h