Avoid atomic overhead in Servo_Element_IsDisplayNone.

This reduces time spent in NoteDirtyElement by about 15% in the testcase. Even
though the subsequent patch will cause us to call Servo_Element_IsDisplayNone less
for this particular testcase, it could still be hot on other testcases, and so
it's worth optimizing.

MozReview-Commit-ID: 3F3Zfp48dDW

ports/geckolib/glue.rs

@@ -1024,8 +1024,13 @@ pub extern "C" fn Servo_Element_GetPseudoComputedValues(element: RawGeckoElement
 #[no_mangle]
 pub extern "C" fn Servo_Element_IsDisplayNone(element: RawGeckoElementBorrowed) -> bool {
     let element = GeckoElement(element);
-    let data = element.borrow_data().expect("Invoking Servo_Element_IsDisplayNone on unstyled element");
-    data.styles.is_display_none()
+    let data = element.get_data().expect("Invoking Servo_Element_IsDisplayNone on unstyled element");
+
+    // This function is hot, so we bypass the AtomicRefCell. It would be nice to also assert that
+    // we're not in the servo traversal, but this function is called at various intermediate
+    // checkpoints when managing the traversal on the Gecko side.
+    debug_assert!(is_main_thread());
+    unsafe { &*data.as_ptr() }.styles.is_display_none()
 }
 
 #[no_mangle]