Bug 1544345 - Fix perma orange on test_setSinkId.html caused by redeclaring a variable with let. r=Jamie

Differential Revision: https://phabricator.services.mozilla.com/D27464

--HG--
extra : moz-landing-system : lando

dom/media/tests/mochitest/test_setSinkId.html

@@ -33,7 +33,7 @@
     // Have to do this to access normally-preffed off binding methods for some
     // reason.
     // See bug 1544257.
-    let audio = SpecialPowers.wrap(audio);
+    audio = SpecialPowers.wrap(audio);
 
     is(audio.sinkId, "", "Initial value is empty string");