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* Replace base::PostTask*() calls with calling PostTask*() methods
  directly on TaskRunners.

* Use PostTaskAndReplyWithResult() instead of equivalent implementation
  based on PostTaskAndReply().

* Add comment clarifying that IndexedDBContextImpl::IDBTaskRunner() is a
  thread-safe getter.

* Fix incorrect mapping of failure (success = false) to
  blink::mojom::QuotaStatusCode::kOk in DidDeletedIDBData().

The last improvement may result in elevated error rates while attempting
to delete IndexedDB data. This is because this CL correctly propagates
reported errors in a situation where they were ignored before.

Bug: 1016065
Change-Id: Ibde1154fb1c42dc412a40c0ccbcd3cb6b8e3f627
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2588448
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836701}