Bug 1067634: Wrap "will-change" decl in @supports rule, to prevent browser console spam while it's disabled by default. r=heycam

layout/style/full-screen-override.css

@@ -21,6 +21,14 @@
   clip: initial !important;
   transform: initial !important;
   /* FIXME: do we need to worry about 'overflow'? */
-  will-change: initial !important;
+}
+
+/* XXXdholbert This '@supports' wrapper should be removed once we remove
+   support for the pref 'layout.css.will-change.enabled'. (This CSS rule should
+   then be merged with the main rule up above.)  */
+@supports (will-change: initial) {
+  *|*:-moz-full-screen-ancestor {
+    will-change: initial !important;
+  }
 }