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1. Fix a bug when a blocking UI will show multiple times, but only
dismiss once, therefore leaving the scene blocked.
2. Fix a bug when the active scenes are not immediately obscured by the
overlay. Ideally we might not want to show this overlay unless the user
tries to interact with the scene, and initially I hoped to make it so by
observing when the scene becomes active; turns out multiple scenes can
be active at once. We might want to revisit it later.

Bug: 1096062, 1092824,
Change-Id: I978aa855d141e9ba5aa34f7b034a5f284a82ff25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249959
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780291}