Bug 1785329 - Enable animation-composition on Nightly. r=hiro

It seems that making animation shorthand supports animation-composition may be
very tricky, so it's unlikely to include animation-composition into the
shorthand for now, per spec issue:
w3c/csswg-drafts#6946.

WebKit also supports the longhand only on STP (Safari Technology Preview), so
it should be fine to enable the longhand property only on Firefox Nightly,
for experiemental testing.

Differential Revision: https://phabricator.services.mozilla.com/D154934

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

@@ -3300,6 +3300,23 @@ exports.CSS_PROPERTIES = {
       "unset"
     ]
   },
+  "animation-composition": {
+    "isInherited": false,
+    "subproperties": [
+      "animation-composition"
+    ],
+    "supports": [],
+    "values": [
+      "accumulate",
+      "add",
+      "inherit",
+      "initial",
+      "replace",
+      "revert",
+      "revert-layer",
+      "unset"
+    ]
+  },
   "animation-delay": {
     "isInherited": false,
     "subproperties": [

modules/libpref/init/StaticPrefList.yaml

@@ -7716,7 +7716,7 @@
 # Whether the `animation-composition` in css-animations-2 is enabled.
 - name: layout.css.animation-composition.enabled
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 # Is the codepath for using cached scrollbar styles enabled?