cppgc: Fix benign data race in MemberBase

The ctors dispatch between atomic and non-atomic writes; there's no
need for a default initializer.

Bug: chromium:1292728
Change-Id: I2b4c3341ee2d2682ba0113c8366456147ebc717e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3429201Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78884}

include/cppgc/member.h

@@ -26,7 +26,7 @@ class MemberBase {
  protected:
   struct AtomicInitializerTag {};
 
-  MemberBase() = default;
+  MemberBase() : raw_(nullptr) {}
   explicit MemberBase(const void* value) : raw_(value) {}
   MemberBase(const void* value, AtomicInitializerTag) { SetRawAtomic(value); }
 
@@ -46,7 +46,10 @@ class MemberBase {
   void ClearFromGC() const { raw_ = nullptr; }
 
  private:
-  mutable const void* raw_ = nullptr;
+  // All constructors initialize `raw_`. Do not add a default value here as it
+  // results in a non-atomic write on some builds, even when the atomic version
+  // of the constructor is used.
+  mutable const void* raw_;
 };
 
 // The basic class from which all Member classes are 'generated'.