Add position: sticky as supported position value when CSSStickyPosition is enabled.

Source/core/css/parser/CSSParserFastPaths.cpp

Index: Source/core/css/parser/CSSParserFastPaths.cpp
diff --git a/Source/core/css/parser/CSSParserFastPaths.cpp b/Source/core/css/parser/CSSParserFastPaths.cpp
index 83d290a3eaf83ff7eb2fbea7ac560ed83a94e22d..20b122f09a42f7d7ecaeb19062e6b689a2407480 100644
--- a/Source/core/css/parser/CSSParserFastPaths.cpp
+++ b/Source/core/css/parser/CSSParserFastPaths.cpp
@@ -536,8 +536,8 @@ bool CSSParserFastPaths::isValidKeywordPropertyAndValue(CSSPropertyID propertyId
         // none | visiblePainted | visibleFill | visibleStroke | visible |
         // painted | fill | stroke | auto | all | bounding-box
         return valueID == CSSValueVisible || valueID == CSSValueNone || valueID == CSSValueAll || valueID == CSSValueAuto || (valueID >= CSSValueVisiblePainted && valueID <= CSSValueBoundingBox);
-    case CSSPropertyPosition: // static | relative | absolute | fixed
-        return valueID == CSSValueStatic || valueID == CSSValueRelative || valueID == CSSValueAbsolute || valueID == CSSValueFixed;
+    case CSSPropertyPosition: // static | relative | absolute | fixed | sticky
+        return valueID == CSSValueStatic || valueID == CSSValueRelative || valueID == CSSValueAbsolute || valueID == CSSValueFixed || (RuntimeEnabledFeatures::cssStickyPositionEnabled() && valueID == CSSValueSticky);
     case CSSPropertyResize: // none | both | horizontal | vertical | auto
         return valueID == CSSValueNone || valueID == CSSValueBoth || valueID == CSSValueHorizontal || valueID == CSSValueVertical || valueID == CSSValueAuto;
     case CSSPropertyScrollBehavior: // auto | smooth