Split CPDF_PageObjectHolder off from CPDF_PageObjectList

fpdfsdk/src/fpdf_flatten.cpp

Index: fpdfsdk/src/fpdf_flatten.cpp
diff --git a/fpdfsdk/src/fpdf_flatten.cpp b/fpdfsdk/src/fpdf_flatten.cpp
index 413b391eeef93ba5a29d0a35b978657e2c6d3b69..36b8319dcace85d09f5fa6f5750dd8091bca37af 100644
--- a/fpdfsdk/src/fpdf_flatten.cpp
+++ b/fpdfsdk/src/fpdf_flatten.cpp
@@ -42,9 +42,10 @@ FX_BOOL GetContentsRect(CPDF_Document* pDoc,
   pPDFPage->Load(pDoc, pDict, FALSE);
   pPDFPage->ParseContent(nullptr);
 
-  FX_POSITION pos = pPDFPage->GetFirstObjectPosition();
+  FX_POSITION pos = pPDFPage->GetPageObjectList()->GetHeadPosition();
   while (pos) {
-    CPDF_PageObject* pPageObject = pPDFPage->GetNextObject(pos);
+    CPDF_PageObject* pPageObject =
+        pPDFPage->GetPageObjectList()->GetNextObject(pos);
     if (!pPageObject)
       continue;