Disentangle fpdfsave_embeddertest's FPDF_FILEWRITE, use gmock matchers.

testing/test_support.cpp

 // 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.
 
 #include "testing/test_support.h"
 
 #include <stdio.h>
 #include <string.h>
 
 #include "testing/utils/path_service.h"
@@ skipping 154 matching lines @@
                          unsigned long pos,
                          unsigned char* pBuf,
                          unsigned long size) {
   TestLoader* pLoader = static_cast<TestLoader*>(param);
   if (pos + size < pos || pos + size > pLoader->m_Len)
     return 0;
 
   memcpy(pBuf, pLoader->m_pBuf + pos, size);
   return 1;
 }
+
+TestSaver::TestSaver() {
+  FPDF_FILEWRITE::version = 1;
+  FPDF_FILEWRITE::WriteBlock = WriteBlockCallback;
+}
+
+void TestSaver::ClearString() {
+  m_String.clear();
+}
+
+// static
+int TestSaver::WriteBlockCallback(FPDF_FILEWRITE* pFileWrite,
+                                  const void* data,
+                                  unsigned long size) {
+  TestSaver* pThis = static_cast<TestSaver*>(pFileWrite);
+  pThis->m_String.append(static_cast<const char*>(data), size);
+  return 1;
+}