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Sometimes WebAppMetrics is still holding a foreground_web_contents on
shutdown, and thus fails to deregister itself as an observer of the
associated AppBannerManager, which is already gone.
This change adds extra checks to try to prevent getting into this state,
and dumps crash reports if in an unexpected state.

Bug: 1162123
Change-Id: I142e6cc30395decf7a09ca2c6773de8c65865535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620919
Commit-Queue: Glen Robertson <glenrob@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843424}