Derive the step base for an input element as (now) specified.

Source/core/html/forms/BaseDateAndTimeInputType.cpp

Index: Source/core/html/forms/BaseDateAndTimeInputType.cpp
diff --git a/Source/core/html/forms/BaseDateAndTimeInputType.cpp b/Source/core/html/forms/BaseDateAndTimeInputType.cpp
index bf87e948cc0f52c30c0bc49b9912411dd1313c28..33a7ab77c0fbd0ffca314c81c2767aebddd701a1 100644
--- a/Source/core/html/forms/BaseDateAndTimeInputType.cpp
+++ b/Source/core/html/forms/BaseDateAndTimeInputType.cpp
@@ -60,7 +60,7 @@ void BaseDateAndTimeInputType::setValueAsDate(double value, ExceptionState&) con
 
 double BaseDateAndTimeInputType::valueAsDouble() const
 {
-    const Decimal value = parseToNumber(element().value(), Decimal::nan());
+    const Decimal value = parseToNumber(element().value(), String(), Decimal::nan());
     return value.isFinite() ? value.toDouble() : DateComponents::invalidMilliseconds();
 }
 
@@ -103,10 +103,10 @@ bool BaseDateAndTimeInputType::isSteppable() const
     return true;
 }
 
-Decimal BaseDateAndTimeInputType::parseToNumber(const String& source, const Decimal& defaultValue) const
+Decimal BaseDateAndTimeInputType::parseToNumber(const String& source, const String& alternative, const Decimal& defaultValue) const
 {
     DateComponents date;
-    if (!parseToDateComponents(source, &date))
+    if (!parseToDateComponents(source, &date) && !parseToDateComponents(alternative, &date))
         return defaultValue;
     double msec = date.millisecondsSinceEpoch();
     ASSERT(std::isfinite(msec));