Mojo: MultiprocessTestBase -> MultiprocessTestHelper.

Because inheritance sucks.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/190943003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255798 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255798