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|UpdateExistingAccessPointRlz| involves file I/O, so it should be
posted on a task runner that has MayBlock() in its TaskTraits.
|background_task_runner_| serves this purpose.

Bug: 906293
Change-Id: If36215767a0dbcfbad56c0432b16b584f6185376
Reviewed-on: https://chromium-review.googlesource.com/c/1340607Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609428}