[Impeller] Ensure that the auto-release pool is available when the GoldenPlaygroundTestImpl is destructed (#43322)

Author: jason-simmons

impeller/golden_tests/golden_playground_test.h

@@ -52,14 +52,16 @@ class GoldenPlaygroundTest
   ISize GetWindowSize() const;
 
  private:
-  struct GoldenPlaygroundTestImpl;
-  // This is only a shared_ptr so it can work with a forward declared type.
-  std::shared_ptr<GoldenPlaygroundTestImpl> pimpl_;
-
 #if FML_OS_MACOSX
+  // This must be placed first so that the autorelease pool is not destroyed
+  // until the GoldenPlaygroundTestImpl has been destructed.
   fml::ScopedNSAutoreleasePool autorelease_pool_;
 #endif
 
+  struct GoldenPlaygroundTestImpl;
+  // This is only a shared_ptr so it can work with a forward declared type.
+  std::shared_ptr<GoldenPlaygroundTestImpl> pimpl_;
+
   FML_DISALLOW_COPY_AND_ASSIGN(GoldenPlaygroundTest);
 };