Make internal BSTR private in ScopedBstr (#25096)

There are no subclasses of this class in our codebase that would
necessitate a protected member.

third_party/accessibility/base/win/scoped_bstr.h

@@ -83,10 +83,8 @@ class BASE_EXPORT ScopedBstr {
   bool operator==(const ScopedBstr& bstr2) const = delete;
   bool operator!=(const ScopedBstr& bstr2) const = delete;
 
- protected:
-  BSTR bstr_ = nullptr;
-
  private:
+  BSTR bstr_ = nullptr;
   BASE_DISALLOW_COPY_AND_ASSIGN(ScopedBstr);
 };