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Fixes a rare use-after-free crash that could occur when an animating
layout manager was already fading out a view and then that view was
removed from its parent.

Also adds a DCHECK and two regression tests to prevent the problem from
occurring again (the DCHECK covers ALL LayoutManagerBase-derived
layouts, not just AnimatingLayoutManager).

Bug: 1037625
Change-Id: I87f4611a081bad7bb4fcffa30ab60a81502aeb1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981338Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727489}