Refactor minidump test utilities for MinidumpWritable, …

minidump/minidump_exception_writer_test.cc

Index: minidump/minidump_exception_writer_test.cc
diff --git a/minidump/minidump_exception_writer_test.cc b/minidump/minidump_exception_writer_test.cc
index 16cff2481402bb957dc6a82d09d2db169e9b8b84..e1cad612f6120905bb49f9852933e9998eeb8137 100644
--- a/minidump/minidump_exception_writer_test.cc
+++ b/minidump/minidump_exception_writer_test.cc
@@ -44,15 +44,11 @@ void GetExceptionStream(const std::string& file_contents,
   const size_t kFileSize = kContextOffset + sizeof(MinidumpContextX86);
   ASSERT_EQ(file_contents.size(), kFileSize);
 
+  const MINIDUMP_DIRECTORY* directory;
   const MINIDUMP_HEADER* header =
-      reinterpret_cast<const MINIDUMP_HEADER*>(&file_contents[0]);
-
+      MinidumpHeaderAtStart(file_contents, &directory);
   ASSERT_NO_FATAL_FAILURE(VerifyMinidumpHeader(header, 1, 0));
 
-  const MINIDUMP_DIRECTORY* directory =
-      reinterpret_cast<const MINIDUMP_DIRECTORY*>(
-          &file_contents[kDirectoryOffset]);
-
   ASSERT_EQ(kMinidumpStreamTypeException, directory[0].StreamType);
   ASSERT_GE(directory[0].Location.DataSize, sizeof(MINIDUMP_EXCEPTION_STREAM));
   ASSERT_EQ(kExceptionStreamOffset, directory[0].Location.Rva);