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In Executive.kill_process, drop WNOHANG from os.waitpid and use the
default blocking wait instead.

This makes sure we can reliably collect the defunct process, which fixes
a peculiar issue on macOS: when calling os.killpg on a defunct process
group, we get EACCES instead of failing silently, which makes a unit
test flaky on macOS.

Drive-by:
* Update the docstring to reflect the reality (we no longer ignore
  permission errors).

Bug: 1119341
Change-Id: I1efa79d8e610e67cede7d9492a82ba67abc8e0a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459326Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815265}