libservo: Add a WebView::take_screenshot() API and use it for reftests

Co-authored-by: Delan Azabani <dazabani@igalia.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/messaging.rs

@@ -102,6 +102,7 @@ impl MixedMessage {
                 ScriptThreadMessage::PreferencesUpdated(..) => None,
                 ScriptThreadMessage::NoLongerWaitingOnAsychronousImageUpdates(_) => None,
                 ScriptThreadMessage::ForwardKeyboardScroll(id, _) => Some(*id),
+                ScriptThreadMessage::RequestScreenshotReadiness(id) => Some(*id),
             },
             MixedMessage::FromScript(inner_msg) => match inner_msg {
                 MainThreadScriptMsg::Common(CommonScriptMsg::Task(_, _, pipeline_id, _)) => {