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We'd previously set opacity to 0.0 when hiding elements in the
transience manager which makes it tricky to see them when they come
back. With this change, we inform the manager of the desired "opacity
when visible" and it will restore elements to that opacity when
shown again.

Bug: None
Change-Id: I1a941c3032c35adab5afed17a8afc60bd41230f5
Reviewed-on: https://chromium-review.googlesource.com/582069
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488788}