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The crash appears to be an accessibility change event triggered by an
aura::Window visibility change in the middle of a views::Widget close.
I have not been able to reproduce.

AXRemoteHost owns the AXTreeSource and AXTreeSerializer in the remote
app process. Change it to stop sending AX node tree updates to the
browser process earlier in Widget close.

Also add a defensive null check for a missing widget, explicit cleanup
of the AXRemoteHost slightly earlier during remote process shutdown,
and some DCHECKs.

Bug: 869608
Test: added to views_mus_unittests
Change-Id: I5a69557382f457258d619e61342ddeca8a57aaf0
Reviewed-on: https://chromium-review.googlesource.com/1162520Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580719}