Bug 1385478 - Part 1: Use IsRequired/Required() to get the current required state. r=bz

IsRequired() helper function returns the current 'required' state of the
element, that is, whether its required attribute is set or not. This will be
used only for input elements that @required applies.

dom/html/HTMLTextAreaElement.h

@@ -252,7 +252,7 @@ public:
   {
     SetHTMLBoolAttr(nsGkAtoms::readonly, aReadOnly, aError);
   }
-  bool Required()
+  bool Required() const
   {
     return GetBoolAttr(nsGkAtoms::required);
   }