constellation: Pass system theme to new Pipelines (#37132)

Previously, when the theme was set it was only set on currently active
`Window`s. This change makes setting the `Theme` stateful. Now the
`Constellation` tracks what theme is applied to a `WebView` and properly
passes that value to new `Pipeline`s when they are constructed. In
addition, the value is passed to layout when that is constructed as
well.

Testing: this change adds a unit test.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/windowproxy.rs

@@ -329,6 +329,9 @@ impl WindowProxy {
             opener: Some(self.browsing_context_id),
             load_data,
             viewport_details: window.viewport_details(),
+            // Use the current `WebView`'s theme initially, but the embedder may
+            // change this later.
+            theme: window.theme(),
         };
         ScriptThread::process_attach_layout(new_layout_info, document.origin().clone());
         // TODO: if noopener is false, copy the sessionStorage storage area of the creator origin.