style: Expose the traversal kind to the style system.

This way we'll be able to take different paths for the sequential and parallel
traversals in some concrete cases.

This is a preliminar patch to fix bug 1332525.

components/style/traversal.rs

@@ -62,6 +62,23 @@ impl LogBehavior {
     fn allow(&self) -> bool { matches!(*self, MayLog) }
 }
 
+/// The kind of traversals we could perform.
+#[derive(Debug, Copy, Clone)]
+pub enum TraversalDriver {
+    /// A potentially parallel traversal.
+    Parallel,
+    /// A sequential traversal.
+    Sequential,
+}
+
+impl TraversalDriver {
+    /// Returns whether this represents a parallel traversal or not.
+    #[inline]
+    pub fn is_parallel(&self) -> bool {
+        matches!(*self, TraversalDriver::Parallel)
+    }
+}
+
 /// A DOM Traversal trait, that is used to generically implement styling for
 /// Gecko and Servo.
 pub trait DomTraversal<E: TElement> : Sync {
@@ -284,6 +301,14 @@ pub trait DomTraversal<E: TElement> : Sync {
 
     /// Creates a thread-local context.
     fn create_thread_local_context(&self) -> Self::ThreadLocalContext;
+
+    /// Whether we're performing a parallel traversal.
+    ///
+    /// NB: We do this check on runtime. We could guarantee correctness in this
+    /// regard via the type system via a `TraversalDriver` trait for this trait,
+    /// that could be one of two concrete types. It's not clear whether the
+    /// potential code size impact of that is worth it.
+    fn is_parallel(&self) -> bool;
 }
 
 /// Helper for the function below.