cppgc: Fix CustomSpace trait

Expose kSupportsCompaction to be able to refer to it from other traits.

Change-Id: I3a0870853fabfac993eff22886a0a31a52d90055
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2653225
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72372}

include/cppgc/custom-space.h

@@ -48,15 +48,18 @@ class CustomSpaceBase {
 template <typename ConcreteCustomSpace>
 class CustomSpace : public CustomSpaceBase {
  public:
+  /**
+   * Compaction is only supported on spaces that manually manage slots
+   * recording.
+   */
+  static constexpr bool kSupportsCompaction = false;
+
   CustomSpaceIndex GetCustomSpaceIndex() const final {
     return ConcreteCustomSpace::kSpaceIndex;
   }
   bool IsCompactable() const final {
     return ConcreteCustomSpace::kSupportsCompaction;
   }
-
- protected:
-  static constexpr bool kSupportsCompaction = false;
 };
 
 /**