Disentangle fpdfsave_embeddertest's FPDF_FILEWRITE, use gmock matchers.

testing/test_support.h

 // Copyright 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_SUPPORT_H_
 #define TESTING_EMBEDDER_TEST_SUPPORT_H_
 
 #include <stdlib.h>
 #include <memory>
 #include <string>
 
 #include "public/fpdfview.h"
+#include "public/fpdf_save.h"
 
 #ifdef PDF_ENABLE_V8
 #include "v8/include/v8.h"
 #endif  // PDF_ENABLE_V8
 
 namespace pdfium {
 
 // Used with std::unique_ptr to free() objects that can't be deleted.
 struct FreeDeleter {
   inline void operator()(void* ptr) const { free(ptr); }
@@ skipping 32 matching lines @@
   static int GetBlock(void* param,
                       unsigned long pos,
                       unsigned char* pBuf,
                       unsigned long size);
 
  private:
   const char* const m_pBuf;
   const size_t m_Len;
 };
 
+class TestSaver : public FPDF_FILEWRITE {
+ public:
+  TestSaver();
+
+  void ClearString();
+  const std::string& GetString() const { return m_String; }
+
+ private:
+  static int WriteBlockCallback(FPDF_FILEWRITE* pFileWrite,
+                                const void* data,
+                                unsigned long size);
+
+  std::string m_String;
+};
+
 #endif  // TESTING_EMBEDDER_TEST_SUPPORT_H_