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/constellation/browsingcontext.rs

@@ -5,9 +5,8 @@
 use std::collections::{HashMap, HashSet};
 
 use base::id::{BrowsingContextGroupId, BrowsingContextId, PipelineId, WebViewId};
-use euclid::Size2D;
+use embedder_traits::ViewportDetails;
 use log::warn;
-use style_traits::CSSPixel;
 
 use crate::pipeline::Pipeline;
 
@@ -50,8 +49,8 @@ pub struct BrowsingContext {
     /// The top-level browsing context ancestor
     pub top_level_id: WebViewId,
 
-    /// The size of the frame.
-    pub size: Size2D<f32, CSSPixel>,
+    /// The [`ViewportDetails`] of the frame that this [`BrowsingContext`] represents.
+    pub viewport_details: ViewportDetails,
 
     /// Whether this browsing context is in private browsing mode.
     pub is_private: bool,
@@ -85,7 +84,7 @@ impl BrowsingContext {
         top_level_id: WebViewId,
         pipeline_id: PipelineId,
         parent_pipeline_id: Option<PipelineId>,
-        size: Size2D<f32, CSSPixel>,
+        viewport_details: ViewportDetails,
         is_private: bool,
         inherited_secure_context: Option<bool>,
         throttled: bool,
@@ -96,7 +95,7 @@ impl BrowsingContext {
             bc_group_id,
             id,
             top_level_id,
-            size,
+            viewport_details,
             is_private,
             inherited_secure_context,
             throttled,