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Fixes a bug where the unified consent screen is not dismissed when
interrupted by a Siri search on the Recent Tabs page.

- Performs complete view controller cleanup on interrupt.
- Dismisses UnifiedConsentViewController when explicitly required.
- Does not explicitly release SigninCoordinator in scene controller,
this is already handled in the completion callback.

Bug: 1115997
Change-Id: Ib4663fe1136a394020020abc76969761c5e0f170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600835Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840588}