Simplify how we suppress the canvas background color.

Instead of conditionally inserting the item at the bottom of the
BorderBackground() list, just suppress its color.

Differential Revision: https://phabricator.services.mozilla.com/D243317

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1956453
gecko-commit: d1cbe9636c3a2c8034db1e6e5abd0bd9def37a57
gecko-reviewers: mstange

Author: emilio

css/compositing/root-element-background-contain-hidden-crash.html

@@ -0,0 +1,11 @@
+<style>
+* {
+  background-color: red;
+  contain: layout;
+}
+*:first-child {
+  visibility: hidden;
+  background-image: url(data:image/gif;base64,iVBORw0KGgoAAAANSUhEUgAAAGAAAABgCAIAAABt+uBvAAAAAXNSR0IArs4c6QAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9wDGhYvCNVA1EIAAAAZdEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIEdJTVBXgQ4XAAAAjklEQVR42u3QIQEAMAgAsHNNVspTgARY1BZh0ZWP3VcgSJAgQYIECRKEIEGCBAkSJEgQggQJEiRIkCBBCBIkSJAgQYIECUKQIEGCBAkSJAhBggQJEiRIkCAECRIkSJAgQYIEIUiQIEGCBAkShCBBggQJEiRIEIIECRIkSJAgQYIQJEiQIEGCBAlCkCBBdwaeugIthHvZ+AAAAABJRU5ErkJggg==)
+}
+</style>
+<object data="x">