Explicit description of the case of a track being added/removed while…

… recording (#152)

Relates to #151

MediaRecorder.bs

@@ -218,8 +218,21 @@ interface MediaRecorder : EventTarget {
       </ol>
       </li>
 
+      <li>If at any point, a track is added to or removed from the {{MediaRecorder/stream}}'s
+      track set, the UA MUST immediately stop gathering data, discard any data that it has gathered,
+      and queue a task, using the DOM manipulation task source, that runs the following steps:
+      <ol>
+        <li>Set {{state}} to {{inactive}}.</li>
+        <li><a>Fire an error event</a> named {{InvalidModificationError}} at
+        <var>target</var>.</li>
+        <li><a href="#to-fire-a-blob-event">Fire a blob event</a> named
+        <a>dataavailable</a> at <var>target</var> with <var>blob</var>.</li>
+        <li><a>Fire an event</a> named <a>stop</a> at <var>target</var>.</li>
+      </ol>
+      </li>
+
       <li>If the UA at any point is unable to continue gathering data for
-      reasons other than isolation properties, it MUST stop gathering data, and
+      reasons other than isolation properties or stream track set, it MUST stop gathering data, and
       queue a task, using the DOM manipulation task source, that runs the
       following steps:
       <ol>