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Fix a UIWindowScene leak in multiwindow. Improve memory management of
overlays.

Bug: 1090263
Change-Id: Ia5e06566635b182fa33c4976c933cebcf0b5f11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228654
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774646}