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This reverts commit c1405389.

Reason for revert: trying a different approach.

Original change's description:
> [vr] Restore opacity when transient elements are shown again.
> 
> 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}

TBR=vollick@chromium.org,cjgrant@chromium.org,ymalik@chromium.org

Change-Id: Ic3ec7b20a980b42c1048c0d074cff6136c3997c2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/582527Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488822}