Bug 1243857 - Use a fake track in devtools web audio tests. irc-r=jesup

MozReview-Commit-ID: HrPTyPwtuLD

devtools/client/webaudioeditor/test/doc_media-node-creation.html

@@ -18,7 +18,7 @@
       let meNode, msNode, mdNode;
       navigator.getUserMedia = navigator.getUserMedia || navigator.mozGetUserMedia;
 
-      navigator.getUserMedia({ audio: true }, stream => {
+      navigator.getUserMedia({ audio: true, fake: true }, stream => {
         meNode = ctx.createMediaElementSource(audio);
         msNode = ctx.createMediaStreamSource(stream);
         mdNode = ctx.createMediaStreamDestination();