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When the BVC presentation experiment is enabled and the last incognito tab is
closed, transition animations continue to run even after the incognito
BrowserState is destroyed.  Variables that are cleared in
|browserStateDestroyed| must be tested for nullptr before use.

BUG=787821

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic5443690337d311e1ce3e347e563d3111cc06f5e
Reviewed-on: https://chromium-review.googlesource.com/831095
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524626}