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Fix the problem with the DCHECK in
TestBackgroundPageFirstLoadObserver::OnBackgroundHostCreated() being hit
in case when the extension's background page host gets destroyed and
recreated.

The DCHECK was fired because the class didn't expect to observe more
than one host at the same time. The underlying issue was that the class
didn't stop observing the deleted host, leading to the DCHECK hit and a
potential use-after-free (in tests).

Bug: 1090941
Test: stress-test the "CertificateProviderApiTest.LazyBackgroundPage" browser test after applying crrev.com/c/2312817
Change-Id: I91cf8f3bb0126a04b63e3a73d3f39259c621f330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2312799Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791054}