Fix leak in url_blacklist_manager_unittest.cc.

base::WorkerPool isn't flushed in tests, and some of the blacklist tests
post a background task that owns allocated memory. This task never gets flushed
and leak the memory.

This fix uses PostTaskAndReplyWithResult using a scoped_ptr to pass the data
across the threads instead.

BUG=271392
R=pastarmovj@chromium.org
TBR=joaodasilva@chromium.org

Review URL: https://codereview.chromium.org/106093007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241296 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241296