Bug 1341108 - Work around libcubeb backends without multi-channel support. r=jya

MozReview-Commit-ID: H5ROBTMVkkE

--HG--
extra : rebase_source : 70869fd590575f98f515c4b7deb90180399e8588

dom/media/MediaPrefs.h

@@ -90,10 +90,11 @@ private:
   DECL_MEDIA_PREF("accessibility.monoaudio.enable",           MonoAudio, bool, false);
   DECL_MEDIA_PREF("media.resampling.enabled",                 AudioSinkResampling, bool, false);
   DECL_MEDIA_PREF("media.resampling.rate",                    AudioSinkResampleRate, uint32_t, 48000);
-#if defined(ANDROID)
-  DECL_MEDIA_PREF("media.forcestereo.enabled",                AudioSinkForceStereo, bool, true);
-#else
+#if defined(XP_WIN) || defined(XP_DARWIN) || defined(MOZ_PULSEAUDIO)
+  // libcubeb backend implement .get_preferred_channel_layout
   DECL_MEDIA_PREF("media.forcestereo.enabled",                AudioSinkForceStereo, bool, false);
+#else
+  DECL_MEDIA_PREF("media.forcestereo.enabled",                AudioSinkForceStereo, bool, true);
 #endif
   // VideoSink
   DECL_MEDIA_PREF("media.ruin-av-sync.enabled",               RuinAvSync, bool, false);