Bug 1301426 part 1 - implement helper functions; r=jwwang

MozReview-Commit-ID: DL16hNeT8r2

--HG--
extra : rebase_source : 06d9980a4043101d30b12ed7e1af6aa457b0c35c
extra : source : d42e43716fa4bb7f5326e5e377bd0366b75c262f

dom/media/test/mochitest.ini

@@ -497,6 +497,7 @@ support-files =
   owl-short.mp3^headers^
   parser.vtt
   pixel_aspect_ratio.mp4
+  play_promise.js
   r11025_msadpcm_c1.wav
   r11025_msadpcm_c1.wav^headers^
   r11025_s16_c1.wav

dom/media/test/play_promise.js

@@ -0,0 +1,3 @@
+function getNotSupportedFile(name) {
+  return name + ".bad";
+}