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

Source/core/html/forms/NumberInputType.cpp

Index: Source/core/html/forms/NumberInputType.cpp
diff --git a/Source/core/html/forms/NumberInputType.cpp b/Source/core/html/forms/NumberInputType.cpp
index 152df244f7ee0c6a3046a1a7aa7a61b165bb9231..8212eaebcb57e37390ff5cc08c590d53dd4f6ee7 100644
--- a/Source/core/html/forms/NumberInputType.cpp
+++ b/Source/core/html/forms/NumberInputType.cpp
@@ -165,11 +165,12 @@ bool NumberInputType::typeMismatch() const
 StepRange NumberInputType::createStepRange(AnyStepHandling anyStepHandling) const
 {
     DEFINE_STATIC_LOCAL(const StepRange::StepDescription, stepDescription, (numberDefaultStep, numberDefaultStepBase, numberStepScaleFactor));
-    const Decimal stepBase = parseToDecimalForNumberType(element().fastGetAttribute(minAttr), numberDefaultStepBase);
+
+    const Decimal stepBase = parseToNumber(element().fastGetAttribute(minAttr), element().fastGetAttribute(valueAttr), numberDefaultStepBase);
     // FIXME: We should use numeric_limits<double>::max for number input type.
     const Decimal floatMax = Decimal::fromDouble(numeric_limits<float>::max());
-    const Decimal minimum = parseToNumber(element().fastGetAttribute(minAttr), -floatMax);
-    const Decimal maximum = parseToNumber(element().fastGetAttribute(maxAttr), floatMax);
+    const Decimal minimum = parseToNumber(element().fastGetAttribute(minAttr), String(), -floatMax);
+    const Decimal maximum = parseToNumber(element().fastGetAttribute(maxAttr), String(), floatMax);
     const Decimal step = StepRange::parseStep(anyStepHandling, stepDescription, element().fastGetAttribute(stepAttr));
     return StepRange(stepBase, minimum, maximum, step, stepDescription);
 }
@@ -212,8 +213,15 @@ void NumberInputType::handleKeydownEvent(KeyboardEvent* event)
         TextFieldInputType::handleKeydownEvent(event);
 }
 
-Decimal NumberInputType::parseToNumber(const String& src, const Decimal& defaultValue) const
+Decimal NumberInputType::parseToNumber(const String& src, const String& alternative, const Decimal& defaultValue) const
 {
+    if (!alternative.isEmpty()) {
+        Decimal result = parseToDecimalForNumberType(src, Decimal::nan());
+        if (!result.isFinite())
+            result = parseToDecimalForNumberType(alternative, defaultValue);
+
+        return result;
+    }
     return parseToDecimalForNumberType(src, defaultValue);
 }