Added call to event.PreventDefault on html text inputs and textareas.

This change should prevent page scrolling when up/down buttons are pressed.

components/script/dom/htmlinputelement.rs

@@ -656,6 +656,7 @@ impl VirtualMethods for HTMLInputElement {
                         }
                         RedrawSelection => {
                             self.force_relayout();
+                            event.PreventDefault();
                         }
                         Nothing => (),
                     }

components/script/dom/htmltextareaelement.rs

@@ -352,9 +352,11 @@ impl VirtualMethods for HTMLTextAreaElement {
                         }
 
                         self.force_relayout();
+                        event.PreventDefault();
                     }
                     KeyReaction::RedrawSelection => {
                         self.force_relayout();
+                        event.PreventDefault();
                     }
                     KeyReaction::Nothing => (),
                 }