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/webglrenderingcontext.rs

@@ -4680,8 +4680,7 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
 }
 
 impl LayoutCanvasRenderingContextHelpers for LayoutDom<'_, WebGLRenderingContext> {
-    #[allow(unsafe_code)]
-    unsafe fn canvas_data_source(self) -> HTMLCanvasDataSource {
+    fn canvas_data_source(self) -> HTMLCanvasDataSource {
         (*self.unsafe_get()).layout_handle()
     }
 }