Use the mediaControlsButton() helper more in media tests

LayoutTests/media/media-controls-invalid-url.html

Index: LayoutTests/media/media-controls-invalid-url.html
diff --git a/LayoutTests/media/media-controls-invalid-url.html b/LayoutTests/media/media-controls-invalid-url.html
index c9860f88cc566656020a534677cc97084fdee065..ae023ffad9f54c6672da9027a4b85627bac9ac19 100644
--- a/LayoutTests/media/media-controls-invalid-url.html
+++ b/LayoutTests/media/media-controls-invalid-url.html
@@ -19,15 +19,7 @@
 
             function getTimeLineValue()
             {
-                var timeline;
-                var shadowRoot = internals.shadowRoot(video);
-
-                timeline = mediaControlsElement(shadowRoot.firstChild.firstChild, '-webkit-media-controls-timeline');
-
-                if(!timeline)
-                    throw "Faild to find -webkit-media-controls-timeline";
-
-                return timeline.value;
+                return mediaControlsButton(video, "timeline").value;
             }
 
             function error()