crash-reporter: add missing header file from last commit

test_helpers.h

+// Copyright (c) 2010 The Chromium OS 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 _CRASH_REPORTER_TEST_HELPERS_H_
+#define _CRASH_REPORTER_TEST_HELPERS_H_
+
+#include "gtest/gtest.h"
+
+inline void ExpectFileEquals(const char *golden,
+                             const char *file_path) {
+  std::string contents;
+  EXPECT_TRUE(file_util::ReadFileToString(FilePath(file_path),
+                                          &contents));
+  EXPECT_EQ(golden, contents);
+}
+
+#endif  // _CRASH_REPORTER_TEST_HELPERS_H_