MediaVolumeSliderPart is not vertical

LayoutTests/media/media-controls.js

 var captionsButtonElement;
 var captionsButtonCoordinates;
 
 function mediaControlsElement(first, id)
 {
     for (var element = first; element; element = element.nextSibling) {
         // Not every element in the media controls has a shadow pseudo ID, eg. the
         // text nodes for the time values, so guard against exceptions.
         try {
             if (internals.shadowPseudoId(element) == id)
                 return element;
         } catch (exception) { }
 
         if (element.firstChild) {
             var childElement = mediaControlsElement(element.firstChild, id);
             if (childElement)
                 return childElement;
         }
     }
 
     return null;
 }
 
-function mediaControlsButtonCoordinates(element, id)
+function mediaControlsButton(element, id)
 {
     var controlID = "-webkit-media-controls-" + id;
     var button = mediaControlsElement(internals.shadowRoot(element).firstChild, controlID);
     if (!button)
         throw "Failed to find media control element ID '" + id + "'";
+    return button;
+}
 
+function mediaControlsButtonCoordinates(element, id)
+{
+    var button = mediaControlsButton(element, id);
     var buttonBoundingRect = button.getBoundingClientRect();
     var x = buttonBoundingRect.left + buttonBoundingRect.width / 2;
     var y = buttonBoundingRect.top + buttonBoundingRect.height / 2;
     return new Array(x, y);
 }
 
 function mediaControlsButtonDimensions(element, id)
 {
-    var controlID = "-webkit-media-controls-" + id;
-    var button = mediaControlsElement(internals.shadowRoot(element).firstChild, controlID);
-    if (!button)
-        throw "Failed to find media control element ID '" + id + "'";
-
+    var button = mediaControlsButton(element, id);
     var buttonBoundingRect = button.getBoundingClientRect();
     return new Array(buttonBoundingRect.width, buttonBoundingRect.height);
 }
 
 function textTrackDisplayElement(parentElement, id, cueNumber)
 {
     var textTrackContainerID = "-webkit-media-text-track-container";
     var containerElement = mediaControlsElement(internals.shadowRoot(parentElement).firstChild, textTrackContainerID);
 
     if (!containerElement)
@@ skipping 47 matching lines @@
     }
 }
 
 function clickCCButton()
 {
     consoleWrite("*** Click the CC button.");
     eventSender.mouseMoveTo(captionsButtonCoordinates[0], captionsButtonCoordinates[1]);
     eventSender.mouseDown();
     eventSender.mouseUp();
 }