script_layout: Remove script to layout messages (#32081)

Instead of communicating with layout via messages, script can simply
call methods on the layout trait. This simplifies the way that script
communicates with layout and opens the path to sharing more data
structures between the two systems. This is part of a continuing 
series of cleanups after removing the layout thread.

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- [x] These changes do not require tests because they should not change
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components/script/script_thread.rs

@@ -71,8 +71,9 @@ use parking_lot::Mutex;
 use percent_encoding::percent_decode;
 use profile_traits::mem::{self as profile_mem, OpaqueSender, ReportsChan};
 use profile_traits::time::{self as profile_time, profile, ProfilerCategory};
-use script_layout_interface::message::{Msg, ReflowGoal};
-use script_layout_interface::{Layout, LayoutConfig, LayoutFactory, ScriptThreadFactory};
+use script_layout_interface::{
+    Layout, LayoutConfig, LayoutFactory, ReflowGoal, ScriptThreadFactory,
+};
 use script_traits::webdriver_msg::WebDriverScriptCommand;
 use script_traits::CompositorEvent::{
     CompositionEvent, GamepadEvent, IMEDismissedEvent, KeyboardEvent, MouseButtonEvent,
@@ -1210,7 +1211,7 @@ impl ScriptThread {
         };
 
         let _ = window
-            .with_layout(|layout| layout.process(Msg::RegisterPaint(name, properties, painter)));
+            .with_layout(|layout| layout.register_paint_worklet_modules(name, properties, painter));
     }
 
     pub fn push_new_element_queue() {
@@ -2907,9 +2908,7 @@ impl ScriptThread {
             }
 
             debug!("{id}: Shutting down layout");
-            let _ = document.window().with_layout(|layout| {
-                layout.process(Msg::ExitNow);
-            });
+            let _ = document.window().with_layout(|layout| layout.exit_now());
 
             debug!("{id}: Sending PipelineExited message to constellation");
             self.script_sender