Mark CXFA_Document::notify_ as const.

Was hoping to assert non-null. but can be null during tests.

Change-Id: I753e5cc8cfe9975e9bb6cd64c1a013ad8926e76c
Reviewed-on: https://pdfium-review.googlesource.com/c/49770
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/xfa/fxfa/parser/cxfa_document.h b/xfa/fxfa/parser/cxfa_document.h
index 39e3422..a57f7ef 100644
--- a/xfa/fxfa/parser/cxfa_document.h
+++ b/xfa/fxfa/parser/cxfa_document.h
@@ -104,7 +104,7 @@
   std::vector<CXFA_Node*> m_pPendingPageSet;
 
  private:
-  UnownedPtr<CXFA_FFNotify> notify_;
+  UnownedPtr<CXFA_FFNotify> const notify_;
   CXFA_Node* m_pRootNode;
   std::map<uint32_t, CXFA_Node*> m_rgGlobalBinding;
   std::unique_ptr<CFXJSE_Engine> m_pScriptContext;