Enable UnifiedSystemTray by default.

Design doc: http://go/cros-qs-restyling

TEST=ash_unittests,browser_tests
BUG=842079

Change-Id: Ia9eaac6e494fc8ec1b35aaedd9ebf9814890ec57
Reviewed-on: https://chromium-review.googlesource.com/1088458
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566725}

ash/public/cpp/ash_features.cc

@@ -37,7 +37,7 @@ const base::Feature kPersistentWindowBounds{"PersistentWindowBounds",
                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kSystemTrayUnified{"SystemTrayUnified",
-                                       base::FEATURE_DISABLED_BY_DEFAULT};
+                                       base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kTapVisualizerApp{"TapVisualizerApp",
                                       base::FEATURE_ENABLED_BY_DEFAULT};