Ash Tray: Enabled redesigned message center by default.

BUG=976952, 958020

Change-Id: Ied9361ae735c702b3989eb18ab36bf579917bf45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877651Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710209}

ash/public/cpp/ash_features.cc

@@ -91,7 +91,7 @@ const base::Feature kSwapSideVolumeButtonsForOrientation{
     "SwapSideVolumeButtonsForOrientation", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kUnifiedMessageCenterRefactor{
-    "UnifiedMessageCenterRefactor", base::FEATURE_DISABLED_BY_DEFAULT};
+    "UnifiedMessageCenterRefactor", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kEnableBackgroundBlur{"EnableBackgroundBlur",
                                           base::FEATURE_DISABLED_BY_DEFAULT};