Add ./mach build --with-layout-2020

… with corresponding `layout` and `layout_thread` crates,
which for now do nothing.

components/layout_thread_2020/lib.rs

@@ -0,0 +1,59 @@
+/* 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 crossbeam_channel::{Receiver, Sender};
+use euclid::TypedSize2D;
+use gfx::font_cache_thread::FontCacheThread;
+use ipc_channel::ipc::{IpcReceiver, IpcSender};
+use metrics::PaintTimeMetrics;
+use msg::constellation_msg::TopLevelBrowsingContextId;
+use msg::constellation_msg::{BackgroundHangMonitorRegister, PipelineId};
+use net_traits::image_cache::ImageCache;
+use profile_traits::{mem, time};
+use script_traits::LayoutMsg as ConstellationMsg;
+use script_traits::{ConstellationControlMsg, LayoutControlMsg};
+use servo_geometry::DeviceIndependentPixel;
+use servo_url::ServoUrl;
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+pub struct LayoutThread;
+
+impl layout_traits::LayoutThreadFactory for LayoutThread {
+    type Message = script_layout_interface::message::Msg;
+
+    #[allow(unused)]
+    fn create(
+        id: PipelineId,
+        top_level_browsing_context_id: TopLevelBrowsingContextId,
+        url: ServoUrl,
+        is_iframe: bool,
+        chan: (Sender<Self::Message>, Receiver<Self::Message>),
+        pipeline_port: IpcReceiver<LayoutControlMsg>,
+        background_hang_monitor: Box<dyn BackgroundHangMonitorRegister>,
+        constellation_chan: IpcSender<ConstellationMsg>,
+        script_chan: IpcSender<ConstellationControlMsg>,
+        image_cache: Arc<dyn ImageCache>,
+        font_cache_thread: FontCacheThread,
+        time_profiler_chan: time::ProfilerChan,
+        mem_profiler_chan: mem::ProfilerChan,
+        content_process_shutdown_chan: Option<IpcSender<()>>,
+        webrender_api_sender: webrender_api::RenderApiSender,
+        webrender_document: webrender_api::DocumentId,
+        paint_time_metrics: PaintTimeMetrics,
+        busy: Arc<AtomicBool>,
+        load_webfonts_synchronously: bool,
+        initial_window_size: TypedSize2D<u32, DeviceIndependentPixel>,
+        device_pixels_per_px: Option<f32>,
+        dump_display_list: bool,
+        dump_display_list_json: bool,
+        dump_style_tree: bool,
+        dump_rule_tree: bool,
+        relayout_event: bool,
+        nonincremental_layout: bool,
+        trace_layout: bool,
+        dump_flow_tree: bool,
+    ) {
+    }
+}