AllSites: Fix flaky test.

Applies the fix suggested by dbertoni@ on the bug:

Adds a TearDown method which calls
StoragePartition::WaitForDeletionTasksForTesting() to wait for all
deletion tasks to finish and make sure QuotaDatabase does not
run into out of disk space errors.

Bug: 930481
Change-Id: I1d2203d0b6d817ef84ccbda206ab216e54f4fd80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307644


Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791486}