Auto merge of #16833 - MortimerGoro:layout_alignment, r=SimonSapin

Fix unsafe AtomicRefCell<T>  transmutes in Layout component

<!-- Please describe your changes on the following line: -->

Fixes unsafe transmute between `AtomicRefCell<PersistentLayoutData>` and `AtomicRefCell<PartialPersistentLayoutData>` which have different memory alignment in 32 bit archs leading to SEGV crashes. See https://github.com/servo/servo/issues/16817 and https://github.com/servo/servo/pull/16816

mem::align_of values in 32 bit archs (e.g. Android):
```
PersistentLayoutData 8
PersistentLayoutData 4
AtomicRefCell<PersistentLayoutData> 8
AtomicRefCell<PartialPersistentLayoutData> 4
```
mem::align_of values in 64 bit archs
```
PersistentLayoutData 8
PersistentLayoutData 8
AtomicRefCell<PersistentLayoutData> 8
AtomicRefCell<PartialPersistentLayoutData> 8
```

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16817 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script_layout_interface/lib.rs

@@ -50,6 +50,7 @@ use servo_url::ServoUrl;
 use std::sync::atomic::AtomicIsize;
 use style::data::ElementData;
 
+#[repr(C)]
 pub struct PartialPersistentLayoutData {
     /// Data that the style system associates with a node. When the
     /// style system is being used standalone, this is all that hangs
@@ -59,6 +60,9 @@ pub struct PartialPersistentLayoutData {
 
     /// Information needed during parallel traversals.
     pub parallel: DomParallelInfo,
+
+    // Required alignment for safe transmutes between PersistentLayoutData and PartialPersistentLayoutData.
+    _align: [u64; 0]
 }
 
 impl PartialPersistentLayoutData {
@@ -66,6 +70,7 @@ impl PartialPersistentLayoutData {
         PartialPersistentLayoutData {
             style_data: ElementData::new(None),
             parallel: DomParallelInfo::new(),
+            _align: [],
         }
     }
 }