Introduce a process observer that terminates child process if there is
not stdout activity. It can be overridden by an environment variable.

R=apolito@google.com, ehmaldonado@chromium.org

Bug: 1074355
Change-Id: I11de9d29e716587614cf336725c8d4a368a9d61d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2167220Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>