Shouldn't tests be part of the SQL implementation? Why the separate bug?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94

commit bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94
Author: ryansturm <ryansturm@chromium.org>
Date: Mon Nov 07 19:23:53 2016

Adding unit tests for PreviewsOptOutStoreSQL

This tests the behavior of the SQL store backing previews opt outs and
verifeis state is maintained correctly and errors are handled correctly.

BUG= 654464 

Review-Url: https://codereview.chromium.org/2448313002
Cr-Commit-Position: refs/heads/master@{#430341}

[modify] https://crrev.com/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94/components/previews/core/BUILD.gn
[modify] https://crrev.com/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94/components/previews/core/previews_black_list_item.cc
[modify] https://crrev.com/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94/components/previews/core/previews_black_list_item.h
[modify] https://crrev.com/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94/components/previews/core/previews_opt_out_store_sql.cc
[add] https://crrev.com/bdd23f4cc8ff26cae9a28f17f7ed762f99ae3c94/components/previews/core/previews_opt_out_store_sql_unittest.cc