layout: Store scroll offsets in the ScrollTree (#37428)

There are currently five places that scroll offsets are stored:

 - DOM: A set of scroll offsets used for script.
 - Layout: An array of scroll offsets that is used for tracking
   layout-side scroll offsets.
 - Layout: The scroll offsets stored in the `ScrollTree`. These are
   currently unset and unused.
 - Compositor: The scroll offsets stored in the `ScrollTree` mirrored
   from layout.
 - WebRender: The scrolled offsets stored in the WebRender spatial tree.

This change is the first step in combining the first three into the
layout `ScrollTree`. It eliminates the extra array of scroll offsets
stored in layout in favor of the storing them in the `ScrollTree`. A
followup change will eliminate the ones stored in the DOM.

- In addition the `ScrollState` data structure is eliminated as these
are
now stored in a `HashMap` everywhere when passing them via IPC.
- The offsests stored in layout can now never scroll past the boundaries
of the scrolled content.

Testing: This should not change behavior and is thus covered by existing
WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: stevennovaryo <steven.novaryo@gmail.com>

components/script/dom/window.rs

@@ -24,7 +24,7 @@ use canvas_traits::webgl::WebGLChan;
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::{
     DocumentState, LoadData, LoadOrigin, NavigationHistoryBehavior, ScriptToConstellationChan,
-    ScriptToConstellationMessage, ScrollState, StructuredSerializedData, WindowSizeType,
+    ScriptToConstellationMessage, StructuredSerializedData, WindowSizeType,
 };
 use crossbeam_channel::{Sender, unbounded};
 use cssparser::SourceLocation;
@@ -2097,10 +2097,7 @@ impl Window {
         // TODO(mrobinson, #18709): Add smooth scrolling support to WebRender so that we can
         // properly process ScrollBehavior here.
         self.reflow(
-            ReflowGoal::UpdateScrollNode(ScrollState {
-                scroll_id,
-                scroll_offset: Vector2D::new(-x, -y),
-            }),
+            ReflowGoal::UpdateScrollNode(scroll_id, Vector2D::new(-x, -y)),
             can_gc,
         );
     }
@@ -3234,7 +3231,7 @@ fn should_move_clip_rect(clip_rect: UntypedRect<Au>, new_viewport: UntypedRect<f
 fn debug_reflow_events(id: PipelineId, reflow_goal: &ReflowGoal) {
     let goal_string = match *reflow_goal {
         ReflowGoal::UpdateTheRendering => "\tFull",
-        ReflowGoal::UpdateScrollNode(_) => "\tUpdateScrollNode",
+        ReflowGoal::UpdateScrollNode(..) => "\tUpdateScrollNode",
         ReflowGoal::LayoutQuery(ref query_msg) => match *query_msg {
             QueryMsg::ContentBox => "\tContentBoxQuery",
             QueryMsg::ContentBoxes => "\tContentBoxesQuery",