Factor out common StepRange construction code.

Source/core/html/forms/MonthInputType.cpp

Index: Source/core/html/forms/MonthInputType.cpp
diff --git a/Source/core/html/forms/MonthInputType.cpp b/Source/core/html/forms/MonthInputType.cpp
index 5aed90b5e70aa0cd3d20c786cc8d09ac314c63d5..00ac0cb96a5af2d66c1af29b761b4284d04894dc 100644
--- a/Source/core/html/forms/MonthInputType.cpp
+++ b/Source/core/html/forms/MonthInputType.cpp
@@ -108,11 +108,7 @@ StepRange MonthInputType::createStepRange(AnyStepHandling anyStepHandling) const
 {
     DEFINE_STATIC_LOCAL(const StepRange::StepDescription, stepDescription, (monthDefaultStep, monthDefaultStepBase, monthStepScaleFactor, StepRange::ParsedStepValueShouldBeInteger));
 
-    const Decimal stepBase = findStepBase(Decimal::fromDouble(monthDefaultStepBase));
-    const Decimal minimum = parseToNumber(element().fastGetAttribute(minAttr), Decimal::fromDouble(DateComponents::minimumMonth()));
-    const Decimal maximum = parseToNumber(element().fastGetAttribute(maxAttr), Decimal::fromDouble(DateComponents::maximumMonth()));
-    const Decimal step = StepRange::parseStep(anyStepHandling, stepDescription, element().fastGetAttribute(stepAttr));
-    return StepRange(stepBase, minimum, maximum, step, stepDescription);
+    return InputType::createStepRange(anyStepHandling, Decimal::fromDouble(monthDefaultStepBase), Decimal::fromDouble(DateComponents::minimumMonth()), Decimal::fromDouble(DateComponents::maximumMonth()), stepDescription);
 }
 
 Decimal MonthInputType::parseToNumber(const String& src, const Decimal& defaultValue) const