Make mojo::system::Core not own the PlatformSupport.

mojo/edk/embedder/test_embedder.cc

Index: mojo/edk/embedder/test_embedder.cc
diff --git a/mojo/edk/embedder/test_embedder.cc b/mojo/edk/embedder/test_embedder.cc
index defab418e282e155b5401008d3515ce331417279..341262667ead87bc9cdc72d6197fe2860ff100f8 100644
--- a/mojo/edk/embedder/test_embedder.cc
+++ b/mojo/edk/embedder/test_embedder.cc
@@ -54,6 +54,11 @@ bool Shutdown() {
   bool rv = system::internal::ShutdownCheckNoLeaks(internal::g_core);
   delete internal::g_core;
   internal::g_core = nullptr;
+
+  CHECK(internal::g_platform_support);
+  delete internal::g_platform_support;
+  internal::g_platform_support = nullptr;
+
   return rv;
 }