script: Reduce the use of unsafe in LayoutDom (#31979)

Remove the use of unsafe code in the layout wrappers of the DOM. The
main change here is that `unsafe_get()` no longer needs to be an unsafe
method, which allows us to transitively remove or reduce unsafe blocks
from callers. The function itself is not renamed, because it's still
a bit dangerous to start removing the layers of abstraction from actual
DOM nodes.

In addition `init_style_and_opaque_layout_data` can be merged into
`initialize_data`, which removes one more unsafe method.

Finally, a "Safety" section is added to some unsafe methods.

components/script/dom/htmlmediaelement.rs

@@ -2482,11 +2482,14 @@ pub trait LayoutHTMLMediaElementHelpers {
 }
 
 impl LayoutHTMLMediaElementHelpers for LayoutDom<'_, HTMLMediaElement> {
-    #[allow(unsafe_code)]
     fn data(self) -> HTMLMediaData {
-        let media = unsafe { self.unsafe_get() };
         HTMLMediaData {
-            current_frame: media.video_renderer.lock().unwrap().current_frame,
+            current_frame: self
+                .unsafe_get()
+                .video_renderer
+                .lock()
+                .unwrap()
+                .current_frame,
         }
     }
 }