Add a feature flag for Tailored Security observer-based retry mechanism

This changes the usage of kTailoredSecurityDialogRetryMechanism to guard
the development of a cross-platform retry mechanism for the safe
browsing tailored security integration.

Since we still need a flag to guard an observer-based retry mechanism,
this change adds a new flag, called
kTailoredSecurityDialogObserverBasedRetryMechanism, to use for that
feature.

Bug: 1416593
Change-Id: I4c44f83a5e3becd3e5613a3fa52b06c1c5afddb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348653


Commit-Queue: Javier Castro <jacastro@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118909}