Only hanging process recorded so far is gsutil. Sending sigterm doesn't
result in successfully terminating the process. Instead, use sigkill.

R=gavinmak@google.com

Bug: 1255228
Change-Id: Iad354284422676df804c68cb6f18c7db723bf862
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3232250
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>