Use RegistryOverrideManager properly in RLZ tests.

Previously, rlz/test/rlz_test_helpers.cc and
chrome/browser/rlz/rlz_unittest.cc each did somewhat different things to
prepare the registry for tests. Now, rlz_test_helpers.cc does the prep
the right way using RegistryOverrideManager and rlz_unittests.cc uses
rlz_test_helpers.cc.

This unblocks the refactor in r234367 that was reverted in r234627.

BUG=314800
R=rogerta@chromium.org, tommycli@chromium.org

Review URL: https://codereview.chromium.org/63153009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235090 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235090