This is in line with the other new minidump_*_test_util files, namely,
the “context” and “memory” ones. minidump_file_writer_test_util.*
provides VerifyMinidumpHeader(), which is used to validate the
MINIDUMP_HEADER at the beginning of a minidump file, as written by
MinidumpFileWriter.
TEST=minidump_test
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/643643003