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/shared/layout/lib.rs

@@ -360,7 +360,7 @@ pub type IFrameSizes = FnvHashMap<BrowsingContextId, IFrameSize>;
 bitflags! {
     /// Conditions which cause a [`Document`] to need to be restyled during reflow, which
     /// might cause the rest of layout to happen as well.
-    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+    #[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
     pub struct RestyleReason: u16 {
         const StylesheetsChanged = 1 << 0;
         const DOMChanged = 1 << 1;