Fix flakes in SiteDataCache tests

LevelDBSiteDataStore racily writes g_use_in_memory_db_for_testing on
the main thread and reads it on a background thread. This can cause
errors on TSAN bots. It's just coincidence that all the tests currently
have pauses that avoid a race.

SiteDataCacheFacadeBrowserTest deletes a SiteDataReader inside a
callback that the SiteDataReader invokes, while the object being
deleted is still on the stack. This can cause failures on ASAN bots.

Bug: 40755583
Change-Id: I50bc399996fe24d20ca0490579212f6f17b23b40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5426315


Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Commit-Queue: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1283383}