CPDF_Document::LoadPattern() and friends always have a valid matrix.

core/fpdfapi/fpdf_page/include/cpdf_form.h

 // Copyright 2016 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef CORE_FPDFAPI_FPDF_PAGE_INCLUDE_CPDF_FORM_H_
 #define CORE_FPDFAPI_FPDF_PAGE_INCLUDE_CPDF_FORM_H_
 
 #include "core/fpdfapi/fpdf_page/include/cpdf_pageobjectholder.h"
 
 class CPDF_Document;
 class CPDF_Dictionary;
 class CPDF_Stream;
 class CPDF_AllStates;
 class CFX_Matrix;
 class CPDF_Type3Char;
 
 class CPDF_Form : public CPDF_PageObjectHolder {
  public:
   CPDF_Form(CPDF_Document* pDocument,
             CPDF_Dictionary* pPageResources,
             CPDF_Stream* pFormStream,
             CPDF_Dictionary* pParentResources = NULL);
 
   ~CPDF_Form();
 
-  void StartParse(CPDF_AllStates* pGraphicStates,
-                  CFX_Matrix* pParentMatrix,
-                  CPDF_Type3Char* pType3Char,
-                  int level = 0);
-
   void ParseContent(CPDF_AllStates* pGraphicStates,
-                    CFX_Matrix* pParentMatrix,
+                    const CFX_Matrix* pParentMatrix,
                     CPDF_Type3Char* pType3Char,
                     int level = 0);
 
   CPDF_Form* Clone() const;
+
+ private:
+  void StartParse(CPDF_AllStates* pGraphicStates,
+                  const CFX_Matrix* pParentMatrix,
+                  CPDF_Type3Char* pType3Char,
+                  int level = 0);
 };
 
 #endif  // CORE_FPDFAPI_FPDF_PAGE_INCLUDE_CPDF_FORM_H_