Bug 1270310 - Part 3: Make string assignment in HTMLInputElement::GetValueInternal fallible. r=smaug

This makes the string assignment fallible and also adds checks for the return
value from GetValueInternal and GetValue.

dom/html/HTMLInputElement.h

@@ -632,7 +632,7 @@ public:
     SetHTMLAttr(nsGkAtoms::value, aValue, aRv);
   }
 
-  // XPCOM GetValue() is OK
+  void GetValue(nsAString& aValue, ErrorResult& aRv);
   void SetValue(const nsAString& aValue, ErrorResult& aRv);
 
   Nullable<Date> GetValueAsDate(ErrorResult& aRv);