Issue 991 - Subtitles Fail in Safari

It appears there are different behaviours for text cues on
safari compared to other browsers. We were using "auto" for
position, but Safari does not support setting that value for
position. The spec says that the default value is "auto", so
by not setting it, we can assume that it will be "auto".

Closes #991
Closes #1012

Change-Id: Ibedc5f01f72138df85b911bb5112d40f2f327ad3

lib/text/simple_text_displayer.js

@@ -179,6 +179,7 @@ shaka.text.SimpleTextDisplayer.prototype.convertToTextTrackCue_ =
   }
 
   var Cue = shaka.text.Cue;
+  /** @type {VTTCue} */
   var vttCue = new VTTCue(shakaCue.startTime,
                           shakaCue.endTime,
                           shakaCue.payload);
@@ -193,14 +194,13 @@ shaka.text.SimpleTextDisplayer.prototype.convertToTextTrackCue_ =
   vttCue.size = shakaCue.size;
   try {
     // Safari 10 seems to throw on align='center'.
-    // Catch any exception and let the workaround below take care of it.
     vttCue.align = shakaCue.textAlign;
   } catch (exception) {}
 
   if (shakaCue.textAlign == 'center' && vttCue.align != 'center') {
-    // Workaround for a Chrome bug http://crbug.com/663797
-    // Chrome does not support align = 'center'
-    vttCue.position = 'auto';
+    // We want vttCue.position = 'auto'. By default, |position| is set to
+    // "auto". If we set it to "auto" safari will throw an exception, so we
+    // must rely on the default value.
     vttCue.align = 'middle';
   }