Fix null crash in CheckTrailer.

testing/embedder_test.h

 // Copyright (c) 2015 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.
 
 #ifndef TESTING_EMBEDDER_TEST_H_
 #define TESTING_EMBEDDER_TEST_H_
 
 #include <string>
 
 #include "../core/include/fxcrt/fx_system.h"
@@ skipping 47 matching lines @@
   // Load a specific page of the open document.
   virtual FPDF_PAGE LoadPage(int page_number, FPDF_FORMHANDLE form);
 
   // Convert a loaded page into a bitmap.
   virtual FPDF_BITMAP RenderPage(FPDF_PAGE page, FPDF_FORMHANDLE form);
 
   // Relese the resources obtained from LoadPage(). Further use of |page|
   // is prohibited after this call is made.
   virtual void UnloadPage(FPDF_PAGE page, FPDF_FORMHANDLE form);
 
- private:
+ protected:
   FPDF_DOCUMENT document_;
   FPDF_AVAIL avail_;
   FX_DOWNLOADHINTS hints_;
   FPDF_FILEACCESS file_access_;
   FX_FILEAVAIL file_avail_;
   v8::StartupData natives_;
   v8::StartupData snapshot_;
   TestLoader* loader_;
   size_t file_length_;
   char* file_contents_;
 };
 
 #endif  // TESTING_EMBEDDER_TEST_H_