Make second param for TestChildProcess optional.
Make second param for TestChildProcess optional.

This fixes a regression intoduced in crrev.com/1826223004 as the
TestChildProcess function is used by several other tests.

BUG=599545
TEST=sbox_integration_tests on Windows 10.0.10586

Review URL: https://codereview.chromium.org/1847213002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384671}

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