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Delegate the decision to blur the shelf to the shelf layout manager,
and do not blur it unless there is an active session (e.g. login
screen).

Cache whether the background is currently blurred to avoid unnecessary
updates.

Bug: 883558
Change-Id: I6bc881f8133a0363260f88efd4225fb072009fc0
Reviewed-on: https://chromium-review.googlesource.com/1227813Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592978}