Bug 1568446 - Don't insta-free in UniqueArc::assume_init. r=SimonSapin

Also sent to https://github.com/servo/servo/pull/23843.

servo/components/servo_arc/lib.rs

@@ -144,7 +144,7 @@ impl<T> UniqueArc<mem::MaybeUninit<T>> {
     #[inline]
     pub unsafe fn assume_init(this: Self) -> UniqueArc<T> {
         UniqueArc(Arc {
-            p: this.0.p.cast(),
+            p: mem::ManuallyDrop::new(this).0.p.cast(),
             phantom: PhantomData,
         })
     }