stylo: Devirtualize nsIAtom refcounting.

Bug: 1362338
Reviewed-by: froydnj
MozReview-Commit-ID: 3q5rz3L8quQ

components/style/gecko_string_cache/mod.rs

@@ -10,7 +10,7 @@ use gecko_bindings::bindings::Gecko_AddRefAtom;
 use gecko_bindings::bindings::Gecko_Atomize;
 use gecko_bindings::bindings::Gecko_Atomize16;
 use gecko_bindings::bindings::Gecko_ReleaseAtom;
-use gecko_bindings::structs::nsIAtom;
+use gecko_bindings::structs::{nsIAtom, nsIAtom_AtomKind};
 use nsstring::{nsAString, nsString};
 use precomputed_hash::PrecomputedHash;
 use std::ascii::AsciiExt;
@@ -149,7 +149,7 @@ impl WeakAtom {
     #[inline]
     pub fn is_static(&self) -> bool {
         unsafe {
-            (*self.as_ptr()).mIsStatic() != 0
+            (*self.as_ptr()).mKind() == nsIAtom_AtomKind::StaticAtom as u32
         }
     }