constellation: Stop assuming that the viewport is shared by all WebViews (#36312)

The `Constellation` previously held a `window_size` member, but this
assumes that all `WebView`s have the same size. This change removes that
assumption as well as making sure that all `WebView`s pass their size
and HiDIP scaling to the `Constellation` when they are created.

In addition

- `WindowSizeData` is renamed to `ViewportDetails`, as it was
holding more than just the size and it didn't necessarily correspond to
  a "window." It's used for tracking viewport data, whether for an
  `<iframe>` or the main `WebView` viewport.
- `ViewportDetails` is stored more consistently so that conceptually an
  `<iframe>` can also have its own HiDPI scaling. This isn't something
  we necessarily want, but it makes everything conceptually simpler.

The goal with this change is to work toward allowing per-`WebView` HiDPI
scaling and sizing. There are still some corresponding changes in the
compositor to make that happen, but they will in a subsequent change.

Testing: This is covered by existing tests. There should be no behavior
changes.
Fixes: This is part of #36232.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/intersectionobserver.rs

@@ -444,7 +444,7 @@ impl IntersectionObserver {
                 // > (note that this processing step can only be reached if the document is fully active).
                 // TODO: viewport should consider native scrollbar if exist. Recheck Servo's scrollbar approach.
                 document.map(|document| {
-                    let viewport = document.window().window_size().initial_viewport;
+                    let viewport = document.window().viewport_details().size;
                     Rect::from_size(Size2D::new(
                         Au::from_f32_px(viewport.width),
                         Au::from_f32_px(viewport.height),