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We now have a general implementation for dedicated TaskRunners for
OSCrypt's backends. The custom threading implementation for
KeyStorageKeyring can be removed and replaced with the general one.

Bug: 782851
Change-Id: I61b0df8568504d4160bbcce7301e99fbdb86609a
Reviewed-on: https://chromium-review.googlesource.com/804094Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521014}