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/constellation_webview.rs

@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use base::id::BrowsingContextId;
+use embedder_traits::Theme;
+
+use crate::session_history::JointSessionHistory;
+
+/// The `Constellation`'s view of a `WebView` in the embedding layer. This tracks all of the
+/// `Constellation` state for this `WebView`.
+pub(crate) struct ConstellationWebView {
+    /// The currently focused browsing context in this webview for key events.
+    /// The focused pipeline is the current entry of the focused browsing
+    /// context.
+    pub focused_browsing_context_id: BrowsingContextId,
+
+    /// The joint session history for this webview.
+    pub session_history: JointSessionHistory,
+
+    /// The [`Theme`] that this [`ConstellationWebView`] uses. This is communicated to all
+    /// `ScriptThread`s so that they know how to render the contents of a particular `WebView.
+    theme: Theme,
+}
+
+impl ConstellationWebView {
+    pub(crate) fn new(focused_browsing_context_id: BrowsingContextId) -> Self {
+        Self {
+            focused_browsing_context_id,
+            session_history: JointSessionHistory::new(),
+            theme: Theme::Light,
+        }
+    }
+
+    /// Set the [`Theme`] on this [`ConstellationWebView`] returning true if the theme changed.
+    pub(crate) fn set_theme(&mut self, new_theme: Theme) -> bool {
+        let old_theme = std::mem::replace(&mut self.theme, new_theme);
+        old_theme != self.theme
+    }
+
+    /// Get the [`Theme`] of this [`ConstellationWebView`].
+    pub(crate) fn theme(&self) -> Theme {
+        self.theme
+    }
+}