Make TestRLZTrackerDelegate's SequencedWorkerPool multi-threaded.

BUG=646443

Review-Url: https://codereview.chromium.org/2340063003
Cr-Commit-Position: refs/heads/master@{#418674}

components/rlz/rlz_tracker_unittest.cc

@@ -34,7 +34,7 @@ namespace {
 class TestRLZTrackerDelegate : public RLZTrackerDelegate {
  public:
   TestRLZTrackerDelegate()
-      : worker_pool_owner_(1, "TestRLZTracker"),
+      : worker_pool_owner_(2, "TestRLZTracker"),
         request_context_getter_(new net::TestURLRequestContextGetter(
             base::ThreadTaskRunnerHandle::Get())) {}