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* Change ShieldWidget's layer to NOW_DRAWN because it's always transparent.
* Remove layer on image_view/title_view
* Create an layer for unsnappable lable on demand.

Bug: 897870
Test: no functional change. Updated the unit test.
Change-Id: I23093ade07bc943ad16572d2704b78b13883c3bd
Reviewed-on: https://chromium-review.googlesource.com/c/1295335
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601859}