Unsuppress bug_172884.

It hasn't been hit in the memory of
http://chromium-build-logs.appspot.com/suppression_query?suppression_query=bug_172884

NOTRY=true
BUG=172884

Review URL: https://codereview.chromium.org/25365002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226236 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226236