[Telemetry] Terminate any dangling subprocesses
[Telemetry] Terminate any dangling subprocesses

This is designed to prevent deadlocks on the buildbots.

I couldn't figure out a way to kill them without killing the parent as well. I
also haven't looked into a Windows equivalent yet.

BUG=357059

Review URL: https://codereview.chromium.org/301313007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@274036 0039d316-1c4b-4281-b951-d872f2087c98

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