Enable BrowserScheduler.RedirectHistoryService experiment on trunk.

BUG=661143
Committed: https://crrev.com/4aa30591e0953da7e0c54ffdb91856e496d16b5f
Cr-Commit-Position: refs/heads/master@{#440418}

Reverted: https://crrev.com/2885403f92c9ab0a8769c2c41e859502da5a7e6f
Cr-Commit-Position: refs/heads/master@{#440479}
Cause of revert fixed in https://codereview.chromium.org/2611053003/.

Review-Url: https://codereview.chromium.org/2592813002
Cr-Commit-Position: refs/heads/master@{#441997}

testing/variations/fieldtrial_testing_config.json

@@ -214,6 +214,7 @@
                 {
                     "name": "Default",
                     "params": {
+                        "RedirectHistoryService": "true",
                         "RedirectSequencedWorkerPools": "true"
                     }
                 },