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Wait for background I/O tasks to complete before destroying TestingProfiles.

The various SpellcheckCustomDictionaryTests post background tasks to
update the dictionary file. Allow these tasks to complete at TearDown
before destroying the TestingProfiles so that the directories on disk
aren't being removed while background threads are trying to write to
them.

This CL unblocks making base::CreateAndOpenTemporaryFileInDir DCHECK if
the target directory doesn't exist.

BUG=1075917
R=groby@chromium.org

Change-Id: I25b3317447f0928504e3315caa6b215410030c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2193111
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Rachel Blum <groby@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802089}