Implement 'scroll-behavior' parsing

Source/core/rendering/style/RenderStyle.h

Index: Source/core/rendering/style/RenderStyle.h
diff --git a/Source/core/rendering/style/RenderStyle.h b/Source/core/rendering/style/RenderStyle.h
index 1f1e23cfcb28c654a76f8d3e797ea3c56a31c771..a8fbae49e3b1169570b43afa4fda82d059a119c7 100644
--- a/Source/core/rendering/style/RenderStyle.h
+++ b/Source/core/rendering/style/RenderStyle.h
@@ -64,6 +64,7 @@
 #include "platform/geometry/RoundedRect.h"
 #include "platform/graphics/Color.h"
 #include "platform/graphics/GraphicsTypes.h"
+#include "platform/scroll/ScrollableArea.h"
 #include "platform/text/TextDirection.h"
 #include "platform/text/UnicodeBidi.h"
 #include "platform/transforms/TransformOperations.h"
@@ -949,6 +950,8 @@ public:
     TouchAction touchAction() const { return static_cast<TouchAction>(rareNonInheritedData->m_touchAction); }
     TouchActionDelay touchActionDelay() const { return static_cast<TouchActionDelay>(rareInheritedData->m_touchActionDelay); }
 
+    ScrollBehavior scrollBehavior() const { return static_cast<ScrollBehavior>(rareNonInheritedData->m_scrollBehavior); }
+
 // attribute setter methods
 
     void setDisplay(EDisplay v) { noninherited_flags._effectiveDisplay = v; }
@@ -1378,6 +1381,8 @@ public:
     void setTouchAction(TouchAction t) { SET_VAR(rareNonInheritedData, m_touchAction, t); }
     void setTouchActionDelay(TouchActionDelay t) { SET_VAR(rareInheritedData, m_touchActionDelay, t); }
 
+    void setScrollBehavior(ScrollBehavior b) { SET_VAR(rareNonInheritedData, m_scrollBehavior, b); }
+
     const SVGRenderStyle* svgStyle() const { return m_svgStyle.get(); }
     SVGRenderStyle* accessSVGStyle() { return m_svgStyle.access(); }
 
@@ -1675,6 +1680,7 @@ public:
     static TouchActionDelay initialTouchActionDelay() { return TouchActionDelayScript; }
     static ShadowList* initialBoxShadow() { return 0; }
     static ShadowList* initialTextShadow() { return 0; }
+    static ScrollBehavior initialScrollBehavior() { return ScrollBehaviorInstant; }
 
     // The initial value is 'none' for grid tracks.
     static Vector<GridTrackSize> initialGridDefinitionColumns() { return Vector<GridTrackSize>(); }