Introduce disable-shared-dictionary-storage-cleanup-for-testing flag

There is a logic implemented within the network service that cleans up
invalid shared dictionaries when an inconsistency exists between the
database storing the shared dictionary metadata and the disk cache
storing the shared dictionary binaries.

This logic is causing flakiness in a browser test that verifies that a
UseErrorDictionaryLoadFailure error is output to DevTools when a shared
dictionary binary stored in the disk cache is corrupted. To address
this, this CL introduces a flag to disable this cleanup logic and sets
it in the SetUpCommandLine method of
SharedDictionaryDevToolsBrowserTest.

Fixed: 365565147
Change-Id: Ia0122c67b0443a53e3b716ec6774f05d066cfca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5882280
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Patrick Meenan <pmeenan@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359802}