Make VTTCue#endTime be an unrestricted double (#493)

Allow endTime for VTTCue to be set to Infinity to represent unbounded text track cues.

Author: rjksmith

index.bs

@@ -54,6 +54,7 @@ urlPrefix: https://html.spec.whatwg.org/multipage/
             text: rules for extracting the chapter title
             text: text track cue start time
             text: text track cue end time
+            text: unbounded text track cue
             text: expose a user interface to the user
             text: text track cue order
             text: honor user preferences for automatic text track selection
@@ -5546,7 +5547,7 @@ enum PositionAlignSetting { "line-left", "center", "line-right", "auto" };
 enum AlignSetting { "start", "center", "end", "left", "right" };
 [Exposed=Window]
 interface VTTCue : TextTrackCue {
-  constructor(double startTime, double endTime, DOMString text);
+  constructor(double startTime, unrestricted double endTime, DOMString text);
   attribute VTTRegion? region;
   attribute DirectionSetting vertical;
   attribute boolean snapToLines;
@@ -5568,7 +5569,8 @@ interface VTTCue : TextTrackCue {
  <dd>
   <p>Returns a new {{VTTCue}} object, for use with the {{TextTrack/addCue()}} method.</p>
   <p>The |startTime| argument sets the <a>text track cue start time</a>.</p>
-  <p>The |endTime| argument sets the <a>text track cue end time</a>.</p>
+  <p>The |endTime| argument sets the <a>text track cue end time</a>. In the case of the value being
+  positive Infinity, the {{VTTCue}} object represents an <a>unbounded text track cue</a>.</p>
   <p>The |text| argument sets the <a>cue text</a>.</p>
  </dd>
 
@@ -5689,11 +5691,14 @@ interface VTTCue : TextTrackCue {
 
  <li><p>Create a new <a>WebVTT cue</a>. Let |cue| be that <a>WebVTT cue</a>.</p></li>
 
- <li><p>Let |cue|'s <a>text track cue start time</a> be the value of the |startTime| argument,
- interpreted as a time in seconds.</p></li>
+ <li><p>Let |cue|'s <a>text track cue start time</a> be the value of the |startTime|
+ argument.</p></li>
 
- <li><p>Let |cue|'s <a>text track cue end time</a> be the value of the |endTime| argument,
- interpreted as a time in seconds.</p></li>
+ <li><p>If the value of the |endTime| argument is negative Infinity or a Not-a-Number (NaN) value,
+ then throw a <a
+ href="https://tc39.es/ecma262/#sec-native-error-types-used-in-this-standard-typeerror">TypeError
+ </a> exception. Otherwise, let |cue|'s <a>text track cue end time</a> be the value of the |endTime|
+ argument.</p></li>
 
  <li><p>Let |cue|'s <a>cue text</a> be the value of the |text| argument, and let the <a>rules for
  extracting the chapter title</a> be the <a>WebVTT rules for extracting the chapter