script: Clean up Window::force_reflow a little (#37725)

- Move some of the image handling code to a separate function.
 - Move reflow event debugging into layout itself and use the `Debug`
   implementation to print the event.
 - A few other small cleanups

Testing: This should not change behavior and is thus covered by existing
WPT
tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/script/script_thread.rs

@@ -295,9 +295,6 @@ pub struct ScriptThread {
     /// Print Progressive Web Metrics to console.
     print_pwm: bool,
 
-    /// Emits notifications when there is a relayout.
-    relayout_event: bool,
-
     /// Unminify Javascript.
     unminify_js: bool,
 
@@ -964,7 +961,6 @@ impl ScriptThread {
             compositor_api: state.compositor_api,
             profile_script_events: opts.debug.profile_script_events,
             print_pwm: opts.print_pwm,
-            relayout_event: opts.debug.relayout_event,
             unminify_js: opts.unminify_js,
             local_script_source: opts.local_script_source.clone(),
             unminify_css: opts.unminify_css,
@@ -3341,7 +3337,6 @@ impl ScriptThread {
             self.webxr_registry.clone(),
             self.microtask_queue.clone(),
             self.compositor_api.clone(),
-            self.relayout_event,
             self.unminify_js,
             self.unminify_css,
             self.local_script_source.clone(),