style: Make the counters non-atomic counters and merge afterwards.

This was consistently faster in the benchmark (even when counters were disabled,
which was slightly suspicious, but...).

Anyway, it's not really much code, most of it is FFI copy-pasta.

Differential Revision: https://phabricator.services.mozilla.com/D3874

components/style/gecko_bindings/sugar/ownership.rs

@@ -305,6 +305,15 @@ pub struct OwnedOrNull<GeckoType> {
 }
 
 impl<GeckoType> OwnedOrNull<GeckoType> {
+    /// Returns a null pointer.
+    #[inline]
+    pub fn null() -> Self {
+        Self {
+            ptr: ptr::null_mut(),
+            _marker: PhantomData,
+        }
+    }
+
     /// Returns whether this pointer is null.
     #[inline]
     pub fn is_null(&self) -> bool {