Backed out changeset 9b16812e723e (bug 1547750) for telemetry failes at test_subsession_management.py.

toolkit/components/telemetry/tests/marionette/harness/telemetry_harness/runner.py

@@ -29,8 +29,6 @@ class TelemetryTestRunner(BaseMarionetteTestRunner):
                 #   * avoid net access in tests
                 #   * stabilize browser.search.region to avoid an extra subsession (bug 1545207)
                 "browser.search.geoip.url": "data:application/json,{\"country_code\": \"DE\"}",
-                # Disable smart sizing because it changes prefs at startup. (bug 1547750)
-                "browser.cache.disk.smart_size.enabled": False,
                 "toolkit.telemetry.server": "{}/pings".format(SERVER_URL),
                 "toolkit.telemetry.initDelay": 1,
                 "toolkit.telemetry.minSubsessionLength": 0,