Only fire devicechange on 0->1 & 1->0 kind devices pre-gUM.

getusermedia.html

@@ -2724,9 +2724,26 @@ <h3>{{MediaDevices}}</h3>
           <code>false</code>.</p>
         </li>
       </ol>
+      <p>When new media input and/or output devices of a kind are made
+      available where zero devices of that kind were available before, or the
+      lone input and/or output device of a kind becomes unavailable, then if
+      <a>[[\canExposeDeviceInfo]]</a> is <code>false</code>, the User Agent
+      MUST run the following steps in browsing contexts for which the
+      <a>device information can be exposed check</a> returns <code>true</code>,
+      but in no other contexts:</p>
+      <ol>
+        <li>
+          <p>Set <a>[[\storedDeviceList]]</a> to null.</p>
+        </li>
+        <li>
+          <p>Queue a task that fires a simple event named <a>devicechange</a> at the
+          {{MediaDevices}} object.</p>
+        </li>
+      </ol>
       <p>When new media input and/or output devices are made available, or any
       available input and/or output device becomes unavailable, or the system
-      default camera changed, or the system default microphone changed, the User Agent
+      default camera changed, or the system default microphone changed, then
+      if <a>[[\canExposeDeviceInfo]]</a> is <code>true</code>, the User Agent
       MUST run the following steps in browsing contexts for which the
       <a>device information can be exposed check</a> returns <code>true</code>,
       but in no other contexts:</p>