Removes known_crashes.txt SUBSTRING for crbug.com/84872

The reported fix has rolled into chrome:
http://trac.webkit.org/changeset/88216

BUG=84872

TEST=


Review URL: http://codereview.chromium.org/7211012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@90557 0039d316-1c4b-4281-b951-d872f2087c98