Auto merge of #16816 - MortimerGoro:unsafe_layout_data_cast, r=SimonSapin

Fix unsafe casting between PersistentLayoutData and PartialPersistentLayoutData

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PersistentLayoutData and PartialPersistentLayoutData castings in Layout component assume that they have the same base raw address. This is unsafe because field orders is not guaranteed by default, and it's causing some SEGV crashes on Android indeed (https://github.com/servo/servo/issues/16647)

Add a C representation to guarantee that the order of the fields and the safe casting.
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- [x] These changes fix #16647 (github issue number if applicable).

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components/layout/data.rs

@@ -6,6 +6,7 @@ use construct::ConstructionResult;
 use script_layout_interface::PartialPersistentLayoutData;
 
 /// 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.