Setuid sandbox: exit(2) on SIGABRT

The setuid sandbox waits on its one child and then dies
afterwards.

When receiving SIGABRT, instead of dumping core, simply exit the
process. There is no interesting information to be gathered from
knowing that the process is inside waitid(2), one should look at
the child process instead.

This patch is in hope to reduce red herrings.

BUG=334345
TBR=jorgelo@chromium.org

Review URL: https://codereview.chromium.org/166193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251271