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This will ensure any processes which are holding dpkg locks
release them. It is still possible that an init process in the
container will grab the locks again before upgrade_container can run, so
this doesn't ensure dpkg locks are always available, but it should fix the
case of processes lingering and failing to release them.

Bug: 1082075
Change-Id: I138d97b3a3f4ae1a1e4ce13232aef39cc75357a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2217761
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#772594}