I've tried to repro about 20 times using --order=random. Any chance you have a seed that shows the bug?

Testing on Ubuntu. Sorry I don't have a seed you can try but using --repeat-each got both cache-storage.html tests to fail for me (in random and default order).

Tools/Scripts/run-webkit-tests http/tests/cachestorage/ --order=random --repeat-each=10

Slightly more data available in  issue 666991  which tracks importing the upstream version of some of these tests

http/tests/cache-storage/*/cache-storage.html fails because it relies on a cache not existing, but it is implicitly created during the test by a bug in our implementation.

I can't get http/tests/cachestorage/*/cache-add.html to fail.

https://codereview.chromium.org/2520093002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cfe8a0730ee7c02fa8ec082c821b738244b4dd74

commit cfe8a0730ee7c02fa8ec082c821b738244b4dd74
Author: jsbell <jsbell@chromium.org>
Date: Tue Nov 22 20:00:07 2016

Make Cache Storage tests resilient to execution order

BUG= 663871 
R=jkarlin@chromium.org

Review-Url: https://codereview.chromium.org/2520093002
Cr-Commit-Position: refs/heads/master@{#433949}

[modify] https://crrev.com/cfe8a0730ee7c02fa8ec082c821b738244b4dd74/third_party/WebKit/LayoutTests/http/tests/cachestorage/resources/test-helpers.js
[modify] https://crrev.com/cfe8a0730ee7c02fa8ec082c821b738244b4dd74/third_party/WebKit/LayoutTests/http/tests/cachestorage/script-tests/cache-storage.js

Given no repro on http/tests/cachestorage/window/cache-add.html and not seeing it on the bots - http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=http%2Ftests%2Fcachestorage%2Fwindow%2Fcache-add.html - I'm resolving this.