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/constellation/lib.rs

@@ -9,6 +9,7 @@ mod tracing;
 
 mod browsingcontext;
 mod constellation;
+mod constellation_webview;
 mod event_loop;
 mod logging;
 mod pipeline;