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This CL fixes a memory leak that occurs in LocalSafeBrowsingDatabaseManager that
was seen on the Linux Asan bot [1].  The race occurs when a delayed
initialization task happens after the database is closed during shutdown.

[1] https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20ASan%20LSan%20Tests%20%281%29/46741

BUG=None

Change-Id: I45043c243bc6b5af948c8171d72e50f203611395
Reviewed-on: https://chromium-review.googlesource.com/1090235Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565430}