Make CPDF_Font member variables protected.

Change-Id: I5d452ea907f4d243645ddae3512776096a827522
Reviewed-on: https://pdfium-review.googlesource.com/4872
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Nicolás Peña <npm@chromium.org>

core/fpdfapi/page/cpdf_textstate.cpp

diff --git a/core/fpdfapi/page/cpdf_textstate.cpp b/core/fpdfapi/page/cpdf_textstate.cpp
index ed90bd3..6dbad55 100644
--- a/core/fpdfapi/page/cpdf_textstate.cpp
+++ b/core/fpdfapi/page/cpdf_textstate.cpp
@@ -134,7 +134,7 @@
   if (pPageData && m_pFont && !pPageData->IsForceClear())
     pPageData->ReleaseFont(m_pFont->GetFontDict());
 
-  m_pDocument = pFont ? pFont->m_pDocument : nullptr;
+  m_pDocument = pFont ? pFont->GetDocument() : nullptr;
   m_pFont = pFont;
 }