WebVTT: Implement 'best position' from the rendering section

LayoutTests/media/track/track-cue-rendering-best-position-expected.html

Index: LayoutTests/media/track/track-cue-rendering-best-position-expected.html
diff --git a/LayoutTests/media/track/track-cue-rendering-position-auto-expected.html b/LayoutTests/media/track/track-cue-rendering-best-position-expected.html
similarity index 59%
copy from LayoutTests/media/track/track-cue-rendering-position-auto-expected.html
copy to LayoutTests/media/track/track-cue-rendering-best-position-expected.html
index 40ba31560545441d951e6dd12e42ccb0eaca6ec8..cca3bea2359cd7fc507c0e6652776af980815a25 100644
--- a/LayoutTests/media/track/track-cue-rendering-position-auto-expected.html
+++ b/LayoutTests/media/track/track-cue-rendering-best-position-expected.html
@@ -20,17 +20,11 @@ var mediaFile = findMediaFile('video', '../content/test');
 .cue.start {
     text-align: start;
 }
-.cue.middle {
-    text-align: center;
-}
-.cue.end {
-    text-align: end;
-}
 .cue > span {
     font-family: sans-serif;
     background: green;
     color: green;
-    font-size: 50px;
+    font-size: 120px;
     padding: 2px;
 }
 </style>
@@ -38,11 +32,3 @@ var mediaFile = findMediaFile('video', '../content/test');
   <video><script>document.currentScript.parentNode.src = mediaFile;</script></video>
   <div class=video><div class="cue start"><span>XXX</span></div></div>
 </div>
-<div class="container">
-  <video><script>document.currentScript.parentNode.src = mediaFile;</script></video>
-  <div class=video><div class="cue middle"><span>XXX</span></div></div>
-</div>
-<div class="container">
-  <video><script>document.currentScript.parentNode.src = mediaFile;</script></video>
-  <div class=video><div class="cue end"><span>XXX</span></div></div>
-</div>