I created a CL to add an end-to-end test for browsing data deletion:
https://crrev.com/c/1071574/

It currently covers Cookies and various types of site data but additional data should be added, e.g. permissions, autofill, notifications, content settings or payment handlers.

The test shows that multiple leveldb files are not cleaned up properly on Chrome OS. This couldn't be reproduced by manually performing deletions, so it is probably a timing issue of the test.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/46529f81c573f4bdc4093cf8a4820e77fe8bdd1d

commit 46529f81c573f4bdc4093cf8a4820e77fe8bdd1d
Author: Christian Dullweber <dullweber@chromium.org>
Date: Wed May 30 17:03:04 2018

Add test to search for browsing_data on disk

This CL adds a test that visits a website, creates a bunch of storage
for this site, deletes browsing_data and checks whether the hostname
can still be found in the profile directory.

Bug: 846297

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I02d959ef7bd4e7d66f6bbf5558c0515959574a32
Reviewed-on: https://chromium-review.googlesource.com/1071574
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562875}
[modify] https://crrev.com/46529f81c573f4bdc4093cf8a4820e77fe8bdd1d/chrome/browser/browsing_data/browsing_data_remover_browsertest.cc
[modify] https://crrev.com/46529f81c573f4bdc4093cf8a4820e77fe8bdd1d/content/test/data/browsing_data/site_data.html
[modify] https://crrev.com/46529f81c573f4bdc4093cf8a4820e77fe8bdd1d/testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3755faf2434deb23cbfdbe70cb9502eb07005ab8

commit 3755faf2434deb23cbfdbe70cb9502eb07005ab8
Author: Christian Dullweber <dullweber@chromium.org>
Date: Wed Jul 18 10:48:05 2018

Scan LevelDB databases for residues after browsing data deletion.

When deleting browsing data, test that all data has been removed from
level db instances in the profile directory. This improves the existing
test that scans files after a deletion. As LevelDB is using compression
to store it's data, we need to handle it specifically.

Bug: 846297
Change-Id: I93506910430668c735ea6a242352d4497ae19b62
Reviewed-on: https://chromium-review.googlesource.com/1140060
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575995}
[modify] https://crrev.com/3755faf2434deb23cbfdbe70cb9502eb07005ab8/chrome/browser/browsing_data/browsing_data_remover_browsertest.cc

TODOs:
 - Accumulate more data before running a deletion (autofill, videos playback, notifications, ...)
 - Add support for sqlite and other formats that possibly contain non-plaintext data
 - Some databases are already open and the LOCK prevents us from accessing them.
   (Download Service/EntryDB, Sync Data/LevelDB, Thumbnails, data_reduction_proxy_leveldb)

The last part is difficult to solve as a browser_test automatically starts Chrome, which will cause some databases to be opened.