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/element.rs

@@ -323,7 +323,11 @@ impl<'dom, LayoutDataType: LayoutDataTrait> style::dom::TElement
 
     unsafe fn clear_data(&self) {
         if self.get_style_and_opaque_layout_data().is_some() {
-            drop(self.as_node().take_style_and_opaque_layout_data());
+            drop(
+                self.as_node()
+                    .get_jsmanaged()
+                    .take_style_and_opaque_layout_data(),
+            );
         }
     }
 
@@ -758,7 +762,7 @@ impl<'dom, LayoutDataType: LayoutDataTrait> ThreadSafeLayoutElement<'dom>
         self.as_node().type_id()
     }
 
-    unsafe fn unsafe_get(self) -> ServoLayoutElement<'dom, LayoutDataType> {
+    fn unsafe_get(self) -> ServoLayoutElement<'dom, LayoutDataType> {
         self.element
     }