style: Make -moz-binding chrome / UA only.

But enable it in all tests because a lot of them rely on using it in the
style="" attribute for example, or in inline stylesheets, which will no longer
parse this (even in chrome documents), and we don't want to rewrite all the XUL
and XBL tests.

Differential Revision: https://phabricator.services.mozilla.com/D18027

components/style/properties/longhands/box.mako.rs

@@ -602,6 +602,8 @@ ${helpers.predefined_type(
     products="gecko",
     animation_value_type="none",
     gecko_ffi_name="mBinding",
+    gecko_pref="layout.css.moz-binding.content.enabled",
+    enabled_in="chrome",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-binding)",
 )}