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/layout_dom/document.rs

@@ -64,11 +64,11 @@ impl<'ld, LayoutDataType: LayoutDataTrait> ServoLayoutDocument<'ld, LayoutDataTy
     }
 
     pub fn needs_paint_from_layout(&self) {
-        unsafe { self.document.needs_paint_from_layout() }
+        self.document.needs_paint_from_layout()
     }
 
     pub fn will_paint(&self) {
-        unsafe { self.document.will_paint() }
+        self.document.will_paint()
     }
 
     pub fn style_shared_lock(&self) -> &StyleSharedRwLock {