Bug 1138651 - Enable CSS Scroll Snapping by Default on B2G. r=roc

- CSS Scroll Snapping can be enabled on Firefox OS earlier than other
  platforms, as it does not require scrollbar support (Bug 969250)

--HG--
extra : rebase_source : 42ce025dc66cdac411067f7bf72cb94362b0e450

b2g/app/b2g.js

@@ -694,6 +694,9 @@ pref("ui.scrollbarFadeDuration", 200);
 // Scrollbar position follows the document `dir` attribute
 pref("layout.scrollbar.side", 1);
 
+// CSS Scroll Snapping
+pref("layout.css.scroll-snap.enabled", true);
+
 // Enable the ProcessPriorityManager, and give processes with no visible
 // documents a 1s grace period before they're eligible to be marked as
 // background. Background processes that are perceivable due to playing