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This is a re-land with sanity-checks of the kill() outcome removed, so
we can address the effects of process leaks separately from introducing
leak-checking.

Under Windows and Fuchsia the TestLauncher runs test batch processes in
their own jobs, allowing any leaked sub-processes leaked by tests to be
cleaned up.

Under POSIX platforms each batch is run in its own process group, but
the group was only being kill()ed if the test batch failed or crashed.

TestLauncher now proactively kill()s each test batch's process group.

Note that this will still miss processes launched into new process
groups by tests.

Bug: 1094369
Change-Id: I0880f08bd120fc5dd057a54efc3db63173feeca9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359136
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798636}