Add truly const versions of CPDF_Document getters.

Instead of only having CPDF_Dictionary* GetRoot() const, provide
const CPDF_Dictionary* GetRoot() const and CPDF_Dictionary* GetRoot().
Do the same for GetInfo().

Change-Id: I6eae1208d38327fcdc7d0cd75069a01c95f4a92a
Reviewed-on: https://pdfium-review.googlesource.com/11671
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/fpdfformfill.cpp b/fpdfsdk/fpdfformfill.cpp
index 6e142ed..b36c1df 100644
--- a/fpdfsdk/fpdfformfill.cpp
+++ b/fpdfsdk/fpdfformfill.cpp
@@ -721,7 +721,7 @@
     return;
 
   CPDF_Document* pDoc = pFormFillEnv->GetPDFDocument();
-  CPDF_Dictionary* pDict = pDoc->GetRoot();
+  const CPDF_Dictionary* pDict = pDoc->GetRoot();
   if (!pDict)
     return;