Mojo: Fix DCHECK failure in test.

~DataPipe doesn't take the lock (and doesn't have to), which causes lock
assertions to fail.

TBR=davemoore@chromium.org

Review URL: https://codereview.chromium.org/122723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242780 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242780