style: Add ToShmem impl for shared_lock::Locked.

Depends on D17192

Differential Revision: https://phabricator.services.mozilla.com/D17193

components/style/shared_lock.rs

@@ -16,7 +16,10 @@ use std::fmt;
 #[cfg(feature = "servo")]
 use std::mem;
 #[cfg(feature = "gecko")]
+use std::mem::ManuallyDrop;
+#[cfg(feature = "gecko")]
 use std::ptr;
+use to_shmem::{SharedMemoryBuilder, ToShmem};
 
 /// A shared read/write lock that can protect multiple objects.
 ///
@@ -238,6 +241,26 @@ impl<T> Locked<T> {
     }
 }
 
+#[cfg(feature = "gecko")]
+impl<T: ToShmem> ToShmem for Locked<T> {
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> ManuallyDrop<Self> {
+        let guard = self.shared_lock.read();
+        ManuallyDrop::new(Locked {
+            shared_lock: SharedRwLock::read_only(),
+            data: UnsafeCell::new(ManuallyDrop::into_inner(
+                self.read_with(&guard).to_shmem(builder),
+            )),
+        })
+    }
+}
+
+#[cfg(feature = "servo")]
+impl<T: ToShmem> ToShmem for Locked<T> {
+    fn to_shmem(&self, _builder: &mut SharedMemoryBuilder) -> ManuallyDrop<Self> {
+        panic!("ToShmem not supported in Servo currently")
+    }
+}
+
 #[allow(dead_code)]
 mod compile_time_assert {
     use super::{SharedRwLockReadGuard, SharedRwLockWriteGuard};