Add another suppression rule for Bug 51682.

This adds another suppression for Bug 51682 rule started since <http://build.chromium.org/p/chromium.memory/builders/Chromium%20Mac%20(valgrind)/builds/996/steps/memory%20test:%20net/logs/stdio>.

TBR=thestig,timurrrr
BUG=51682
TEST=make the "Chromium Mac (valgrind)" bot green.
Review URL: http://codereview.chromium.org/5893004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69399 0039d316-1c4b-4281-b951-d872f2087c98