XFA: Add a comment regarding the shut down order.

testing/embedder_test.cpp

Index: testing/embedder_test.cpp
diff --git a/testing/embedder_test.cpp b/testing/embedder_test.cpp
index 1f3f6ef54204c91bae03ead44802185b71e0ea36..72b858d2761feb1d3fb91e89a86e4070642ec936 100644
--- a/testing/embedder_test.cpp
+++ b/testing/embedder_test.cpp
@@ -170,6 +170,10 @@ void EmbedderTest::SetUp() {
 void EmbedderTest::TearDown() {
   if (document_) {
     FORM_DoDocumentAAction(form_handle_, FPDFDOC_AACTION_WC);
+
+    // Note: The shut down order here is the reverse of the non-XFA branch
+    // order. Need to work out if this is required, and if it is, the lifetimes
+    // of objects owned by |doc| that |form| reference.
     FPDF_CloseDocument(document_);
     FPDFDOC_ExitFormFillEnvironment(form_handle_);
   }