Disentangle fpdfsave_embeddertest's FPDF_FILEWRITE, use gmock matchers.

testing/test_support.cpp

Index: testing/test_support.cpp
diff --git a/testing/test_support.cpp b/testing/test_support.cpp
index 96e01c5b01cbf04ba989c83e58a6744a557f02d3..da5ea7e9ee5d06126ed8a149c3dca0b90444e52c 100644
--- a/testing/test_support.cpp
+++ b/testing/test_support.cpp
@@ -172,3 +172,21 @@ int TestLoader::GetBlock(void* param,
   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;
+}