GitLab

What:
Use a single LazyThreadPoolSequencedTaskRunner for all conversion
storage operations.

Why:
Conversion Storage doesn't have strong ordering guarantees regarding
databases being torn down and recreated for the same profile.

E.g.

Profile closed -> delete posted to task runner A
Profile opened -> create posted to task runner B

We assume that task runner A will finish executing the delete,
freeing the database prior to task runner B trying to open it. However,
if task runner A has a long running operation already sequenced, this
isn't necessarily true.

Using a unified task runner removes this edge case.

This also removes the need to do any dependency injection which is
discouraged for task runners.

Bug: 1114783
Change-Id: If09d0a9a68ec0d461321744f2734c28b0ccbe068
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490256Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821411}