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/shared/constellation/lib.rs

@@ -55,7 +55,7 @@ pub enum EmbedderToConstellationMessage {
     /// Inform the Constellation that a `WebView`'s [`ViewportDetails`] have changed.
     ChangeViewportDetails(WebViewId, ViewportDetails, WindowSizeType),
     /// Inform the constellation of a theme change.
-    ThemeChange(Theme),
+    ThemeChange(WebViewId, Theme),
     /// Requests that the constellation instruct script/layout to try to layout again and tick
     /// animations.
     TickAnimation(Vec<WebViewId>),