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Regression caused by https://crrev.com/c/1603484. The shelf
background type is not initialized corresponding to the initial
session state. When starting with stub user or an existing user
(crash-n-restore), ShelfLayoutManager thinks it paints the default
background but ShelfWidget actually paints the login background
(i.e. transparent).

Bug: 958214
Test: Run chrome with stub user and observer shelf background.
Change-Id: I6e33e02e4213052ea90ecb0ae94c3dc8b9568dd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604489
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658721}