Refactor minidump test utilities for MinidumpWritable, …

minidump/minidump_thread_writer_test.cc

Index: minidump/minidump_thread_writer_test.cc
diff --git a/minidump/minidump_thread_writer_test.cc b/minidump/minidump_thread_writer_test.cc
index 15f1a07cb212b57aa110d57414796e3d19912551..fb2baff95060c7ffadd1617e5480ce9c3128bfae 100644
--- a/minidump/minidump_thread_writer_test.cc
+++ b/minidump/minidump_thread_writer_test.cc
@@ -44,14 +44,11 @@ void GetThreadListStream(const std::string& file_contents,
 
   ASSERT_GE(file_contents.size(), kThreadsOffset);
 
+  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, kExpectedStreams, 0));
-
-  const MINIDUMP_DIRECTORY* directory =
-      reinterpret_cast<const MINIDUMP_DIRECTORY*>(
-          &file_contents[kDirectoryOffset]);
+  ASSERT_TRUE(directory);
 
   ASSERT_EQ(kMinidumpStreamTypeThreadList, directory[0].StreamType);
   ASSERT_GE(directory[0].Location.DataSize, sizeof(MINIDUMP_THREAD_LIST));