Add helper for computing outset for 'outline'

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 71c132621d8369d5138c3a993d331669364778ad..4a6f1a459751ebf2648af9dc5e5a98e4aa6adf58 100644
--- a/Source/core/style/ComputedStyle.h
+++ b/Source/core/style/ComputedStyle.h
@@ -532,6 +532,7 @@ public:
     bool hasOutline() const { return outlineWidth() > 0 && outlineStyle() > BHIDDEN; }
     EBorderStyle outlineStyle() const { return m_background->outline().style(); }
     OutlineIsAuto outlineStyleIsAuto() const { return static_cast<OutlineIsAuto>(m_background->outline().isAuto()); }
+    int outlineOutset() const;
 
     EOverflow overflowX() const { return static_cast<EOverflow>(noninherited_flags.overflowX); }
     EOverflow overflowY() const { return static_cast<EOverflow>(noninherited_flags.overflowY); }