Upgrade stepUp()/stepDown() implementation to match spec.

Source/core/html/forms/StepRange.h

Index: Source/core/html/forms/StepRange.h
diff --git a/Source/core/html/forms/StepRange.h b/Source/core/html/forms/StepRange.h
index 04288869ad3d8eaa0a82f775993e17f312e4ed9d..f2fdbb2dfde9c3d8ca97736fc12a796d41af74a7 100644
--- a/Source/core/html/forms/StepRange.h
+++ b/Source/core/html/forms/StepRange.h
@@ -71,7 +71,6 @@ public:
     StepRange();
     StepRange(const StepRange&);
     StepRange(const Decimal& stepBase, const Decimal& minimum, const Decimal& maximum, const Decimal& step, const StepDescription&);
-    Decimal acceptableError() const;
     Decimal alignValueForStep(const Decimal& currentValue, const Decimal& newValue) const;
     Decimal clampValue(const Decimal& value) const;
     bool hasStep() const { return m_hasStep; }
@@ -105,6 +104,7 @@ public:
 
 private:
     StepRange& operator =(const StepRange&);
+    Decimal acceptableError() const;
     Decimal roundByStep(const Decimal& value, const Decimal& base) const;
 
     const Decimal m_maximum; // maximum must be >= minimum.