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so fork() doesn't corrupt log files; lets --generate_suppressions 
work even with the hacky valgrind fix that makes anything but --log-file
crash on child of fork().

Also avoid killing user desktop session if zygote dies!

BUG=none, but related to fix for http://crbug.com/15771
TEST=patch valgrind with fork workaround; sh tools/valgrind/chrome_tests.sh --generate_suppressions -t ui logs you out on linux without this.

Review URL: http://codereview.chromium.org/155130

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20024 0039d316-1c4b-4281-b951-d872f2087c98