layout: Stop doing unsafe transmutes between refcell references.

This commit splits the style and layout data in two separate refcells.

These transmutes have been a source of trouble (for example on Android), and
they feel like a hack anyway.

Fixes #16982

components/layout/data.rs

@@ -2,16 +2,32 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use atomic_refcell::AtomicRefCell;
 use construct::ConstructionResult;
-use script_layout_interface::PartialPersistentLayoutData;
+use script_layout_interface::StyleData;
+
+#[repr(C)]
+pub struct StyleAndLayoutData {
+    /// Data accessed by script_layout_interface. This must be first to allow
+    /// casting between StyleAndLayoutData and StyleData.
+    pub style_data: StyleData,
+    /// The layout data associated with a node.
+    pub layout_data: AtomicRefCell<LayoutData>,
+}
+
+impl StyleAndLayoutData {
+    pub fn new() -> Self {
+        Self {
+            style_data: StyleData::new(),
+            layout_data: AtomicRefCell::new(LayoutData::new()),
+        }
+    }
+}
+
 
 /// Data that layout associates with a node.
 #[repr(C)]
-pub struct PersistentLayoutData {
-    /// Data accessed by script_layout_interface. This must be first to allow
-    /// casting between PersistentLayoutData and PartialPersistentLayoutData.
-    pub base: PartialPersistentLayoutData,
-
+pub struct LayoutData {
     /// The current results of flow construction for this node. This is either a
     /// flow or a `ConstructionItem`. See comments in `construct.rs` for more
     /// details.
@@ -29,11 +45,10 @@ pub struct PersistentLayoutData {
     pub flags: LayoutDataFlags,
 }
 
-impl PersistentLayoutData {
+impl LayoutData {
     /// Creates new layout data.
-    pub fn new() -> PersistentLayoutData {
-        PersistentLayoutData {
-            base: PartialPersistentLayoutData::new(),
+    pub fn new() -> LayoutData {
+        Self {
             flow_construction_result: ConstructionResult::None,
             before_flow_construction_result: ConstructionResult::None,
             after_flow_construction_result: ConstructionResult::None,