Implement 'scroll-behavior' parsing

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 293a70fbe2e54c9986ecedbc2b74f6a4ab5d5270..acd801021b6464d542dc4841867b9eb044793abc 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -175,6 +175,7 @@ static const CSSPropertyID staticComputableProperties[] = {
     CSSPropertyPosition,
     CSSPropertyResize,
     CSSPropertyRight,
+    CSSPropertyScrollBehavior,
     CSSPropertySpeak,
     CSSPropertyTableLayout,
     CSSPropertyTabSize,
@@ -2157,6 +2158,8 @@ PassRefPtr<CSSValue> CSSComputedStyleDeclaration::getPropertyCSSValue(CSSPropert
             return valueForPositionOffset(*style, CSSPropertyRight, renderer);
         case CSSPropertyWebkitRubyPosition:
             return cssValuePool().createValue(style->rubyPosition());
+        case CSSPropertyScrollBehavior:
+            return cssValuePool().createValue(style->scrollBehavior());
         case CSSPropertyTableLayout:
             return cssValuePool().createValue(style->tableLayout());
         case CSSPropertyTextAlign: