PartitionAlloc: Always use GigaCage unless BRP is enabled

This is useful for PCScan since it enables the scanning routine to use a
bitmap for super-page lookup.

Bug: 11297512
Change-Id: I1634f0a97a1dd85e3407d80da6cd43e362476062
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627147
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842970}

base/allocator/partition_allocator/partition_root.h

@@ -325,7 +325,11 @@ struct BASE_EXPORT PartitionRoot {
   }
 
   bool UsesGigaCage() const {
-    return features::IsPartitionAllocGigaCageEnabled() && allow_ref_count;
+    return features::IsPartitionAllocGigaCageEnabled()
+#if ENABLE_REF_COUNT_FOR_BACKUP_REF_PTR
+           && allow_ref_count
+#endif
+        ;
   }
 
   ALWAYS_INLINE bool IsScannable() const {