Update type for atomic.

The atomic_uint64_t typedef does not appear to be defined on Ubuntu and
you need to use atomic<uint64_t>

Bug: None

Change-Id: I66f92ae6939723a5ca6f7b80dbb1ad8ab633ce4c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/30726
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/common/RefCounted.h

@@ -35,7 +35,7 @@ class RefCounted {
     virtual void DeleteThis();
 
   private:
-    std::atomic_uint64_t mRefCount;
+    std::atomic<uint64_t> mRefCount;
 };
 
 template <typename T>