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According to UnifiedSystemTray spec, the tray has blurry background.

It is appropriate to use app_list::IsBackgroundBlurEnabled, becuase
app_list is moving from ui/app_list to ash/app_list (go/move-applist),
and also it returns true only on high-end devices which have small
performance impact when the blur is enabled.

Screenshot: http://screen/wUryXYd800Y
Design doc: go/cros-qs-restyling

TEST=out/Release/chrome
--enable-features=EnableBackgroundBlur,SystenTrayUnified
BUG=811641

Change-Id: I270751b734110023261bdb698af33b31fb1af87f
Reviewed-on: https://chromium-review.googlesource.com/970055Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544313}