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Before this patch, the observer/listener for navigation state changes
were implemented using a std::vector. This would lead to null pointer
dereferencing when adding and removing listeners while handling an event
due to std::vector::erase reordering elements (pointers). This patch
uses a checked observer and ObserverList instead to properly handling
adding and removing observers/listeners.

This bug was uncovered while working on the bug linked in this patch.

R=arbesser@google.com

Bug: b/152724265
Change-Id: Ia59bfca5bec5ce326394668eb7d8d62004fc16fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139691
Commit-Queue: Mathias Carlen <mcarlen@chromium.org>
Reviewed-by: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#757021}