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Some PostTask closures do exciting things on destruction like posting
other tasks. Some of them call ThreadTaskRunnerHandle::Get and these where
crashing because it had gone away.  This patch fixes that.

Bug: 933295
Change-Id: I7f48e2743dcebe38f9f6e9f08030d762760fc733
Reviewed-on: https://chromium-review.googlesource.com/c/1482954Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634649}