Bug 1532471 part 2: Enable CSS containment for early beta and earlier. r=emilio

Depends on D23362

Differential Revision: https://phabricator.services.mozilla.com/D23363

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

modules/libpref/init/StaticPrefList.h

@@ -1012,11 +1012,17 @@ VARCACHE_PREF(
 )
 
 // Is support for CSS contain enabled?
+#ifdef EARLY_BETA_OR_EARLIER
+#define PREF_VALUE true
+#else
+#define PREF_VALUE false
+#endif
 VARCACHE_PREF(
   "layout.css.contain.enabled",
    layout_css_contain_enabled,
-  bool, false
+  bool, PREF_VALUE
 )
+#undef PREF_VALUE
 
 // Is steps(jump-*) supported in easing functions?
 VARCACHE_PREF(