part2 : adjust 'left' property for reference cues.

If cue's `position` is smaller than `50`, the cue box would stay left and reduce its box size. If cue's `position` is larger than `50` the cue box would stay right and also reduce its box size [1].

Therefore, for `align_center_position_lt_50-ref` and `align_center_position_lt_50_size_gt_maximum_size-ref`, we should set `left:0` because the box is in the left side.

For `align_center_position_gt_50-ref`, we have to set `right:0`.

[1] https://searchfox.org/mozilla-central/source/dom/media/webvtt/vtt.jsm#641-685

Differential Revision: https://phabricator.services.mozilla.com/D33383

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1556087
gecko-commit: 249be5c1e6432095013d60ebe9834b64d47f4d2a
gecko-integration-branch: autoland
gecko-reviewers: heycam

webvtt/rendering/cues-with-video/processing-model/align_center_position_gt_50-ref.html

@@ -14,7 +14,7 @@
 .cue {
     position: absolute;
     bottom: 0;
-    right: 23px;
+    right: 0px;
     width: 64px;
     text-align: center;
 }

webvtt/rendering/cues-with-video/processing-model/align_center_position_lt_50-ref.html

@@ -14,7 +14,7 @@
 .cue {
     position: absolute;
     bottom: 0;
-    left: 23px;
+    left: 0px;
     right: 0;
     width: 64px;
     text-align: center

webvtt/rendering/cues-with-video/processing-model/align_center_position_lt_50_size_gt_maximum_size-ref.html

@@ -14,7 +14,7 @@
 .cue {
     position: absolute;
     bottom: 0;
-    left: 23px;
+    left: 0px;
     right: 0;
     width: 64px;
     text-align: center;