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This code is subtle. Since killing the host forwarder is a global operation, it
keeps a temp file with the shared pid and process start time so that it knows
whether the forwarder is the correct one that should be shared or a stray one
that should be killed.

This failed because not all of the sharded perf test processes had the same
parent pid. This caused the sharded tests to stomp on the global forwarder.

We didn't notice this previously because we only had narrow sharding, and 3
retries so the flake was papered over. When we added 8-way sharding on
chromium.perf, the flake rose to the level where it appeared every run.

This patch fixes it by using the process group instead of using the parent
process ID as a proxy.

BUG=163503

Review URL: https://codereview.chromium.org/391383005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283683 0039d316-1c4b-4281-b951-d872f2087c98