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

Source/core/style/ComputedStyle.h

Index: Source/core/style/ComputedStyle.h
diff --git a/Source/core/style/ComputedStyle.h b/Source/core/style/ComputedStyle.h
index 3ad70e15ded5c112024236f07aa6924360329742..b3ed24bfca5b7a804496e2265cc3554b43d347d4 100644
--- a/Source/core/style/ComputedStyle.h
+++ b/Source/core/style/ComputedStyle.h
@@ -466,7 +466,7 @@ public:
 
     EPosition position() const { return static_cast<EPosition>(noninherited_flags.position); }
     bool hasOutOfFlowPosition() const { return position() == AbsolutePosition || position() == FixedPosition; }
-    bool hasInFlowPosition() const { return position() == RelativePosition; }
+    bool hasInFlowPosition() const { return position() == RelativePosition || position() == StickyPosition; }
     bool hasViewportConstrainedPosition() const { return position() == FixedPosition; }
     EFloat floating() const { return static_cast<EFloat>(noninherited_flags.floating); }