Mojo: Trivial cleanup -- remove unneeded test fixture in test.

R=sky@chromium.org, sky

Review URL: https://codereview.chromium.org/73243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235255 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235255