Don't create a launcher process when launching a child process in multiprocess unittests.
Don't create a launcher process when launching a child process in multiprocess unittests.

When using MultiProcessTest, we launch a child process with --test-child-process switch. Currently, after the base test launcher code was added, we were actually creating two processes. The first would be a test launcher process, and then that would create the child process. This is unnecessary (since the outer test launcher is enforcing timeouts) and also makes debugging harder (because the inner test launcher times out and kills the child process when debugging it).

Review URL: https://codereview.chromium.org/1313163002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345431}

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