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This is a reland of c1405389
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=cjgrant@chromium.org

Bug: 747633
Change-Id: I340ed94f61f016243dc4495d687cab1d58ea2e67
Reviewed-on: https://chromium-review.googlesource.com/582529Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488833}