Bug 914584. Pref on OMTC for b2g desktop builds. r=fabrice

b2g/app/b2g.js

@@ -256,14 +256,13 @@ pref("ui.dragThresholdY", 25);
 
 // Layers Acceleration.  We can only have nice things on gonk, because
 // they're not maintained anywhere else.
+pref("layers.offmainthreadcomposition.enabled", true);
 #ifndef MOZ_WIDGET_GONK
 pref("dom.ipc.tabs.disabled", true);
-pref("layers.offmainthreadcomposition.enabled", false);
 pref("layers.offmainthreadcomposition.async-animations", false);
 pref("layers.async-video.enabled", false);
 #else
 pref("dom.ipc.tabs.disabled", false);
-pref("layers.offmainthreadcomposition.enabled", true);
 pref("layers.acceleration.disabled", false);
 pref("layers.offmainthreadcomposition.async-animations", true);
 pref("layers.async-video.enabled", true);