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

devtools/shared/css/generated/properties-db.js

@@ -5678,6 +5678,25 @@ exports.CSS_PROPERTIES = {
       "unset"
     ]
   },
+  "contain": {
+    "isInherited": false,
+    "subproperties": [
+      "contain"
+    ],
+    "supports": [],
+    "values": [
+      "content",
+      "inherit",
+      "initial",
+      "layout",
+      "none",
+      "paint",
+      "revert",
+      "size",
+      "strict",
+      "unset"
+    ]
+  },
   "content": {
     "isInherited": false,
     "subproperties": [

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(