Convert mojo::system::ChannelEndpointClient to use our new refcounting stuff (instead of base's).

mojo/edk/system/test_channel_endpoint_client.h

Index: mojo/edk/system/test_channel_endpoint_client.h
diff --git a/mojo/edk/system/test_channel_endpoint_client.h b/mojo/edk/system/test_channel_endpoint_client.h
index 9965cf8441e56553e0797ffa4db1dc49c766b888..592e8ae46371e37389d31d4dfdcaaa05e63c5b6b 100644
--- a/mojo/edk/system/test_channel_endpoint_client.h
+++ b/mojo/edk/system/test_channel_endpoint_client.h
@@ -24,7 +24,7 @@ namespace test {
 
 class TestChannelEndpointClient final : public ChannelEndpointClient {
  public:
-  TestChannelEndpointClient();
+  // Note: Use |MakeRefCounted<TestChannelEndpointClient>()|.
 
   // Initializes with the given port and endpoint.
   void Init(unsigned port, RefPtr<ChannelEndpoint>&& endpoint);
@@ -48,6 +48,9 @@ class TestChannelEndpointClient final : public ChannelEndpointClient {
   void OnDetachFromChannel(unsigned port) override;
 
  private:
+  FRIEND_MAKE_REF_COUNTED(TestChannelEndpointClient);
+
+  TestChannelEndpointClient();
   ~TestChannelEndpointClient() override;
 
   mutable Mutex mutex_;