Bug 1374967 - Part 2: Consider step when deciding whether to show second/millisecond field. r=smaug

We should consider step and step base when deciding whether to show second and
millisecond field, since step and step base can affect the valid time intervals,
and the valid intervals may have second/millisecond part.

MozReview-Commit-ID: H4mJvLTvBOM

dom/html/HTMLInputElement.h

@@ -846,6 +846,13 @@ public:
    */
   void UpdateValidityState();
 
+  /*
+   * The following are called from datetime input box binding to get the
+   * corresponding computed values.
+   */
+  double GetStepAsDouble() { return GetStep().toDouble(); }
+  double GetStepBaseAsDouble() { return GetStepBase().toDouble(); }
+
   HTMLInputElement* GetOwnerNumberControl();
 
   void StartNumberControlSpinnerSpin();