Low Usage Promo: retry instead of failing

Originally if the re-engagement promo tried to show but there were no
active windows in the profile, it would NOTREACHED(). This was useful to
test our assumptions about e.g. startup but is not reasonable for
production, as there are potential race conditions at startup and
shutdown.

This CL changes that NOTREACHED() to a retry that happens on a small
delay. The callback is guarded by both a one-shot timer and a weak
pointer, so should not result in UAFs even if a retry is queued at
browser shutdown.

Since this is a branded-only Chrome feature, include branded trybots:

Cq-Include-Trybots: luci.chrome.try:win64-chrome,win-chrome,mac-chrome,linux-lacros-chrome,linux-chromeos-chrome,linux-chrome,linux-chromeos-chrome-with-lacros

Bug: 340142181
Change-Id: If350d59d03690a8bf06a1bd93d6c9945bec9f31b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5535235
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300196}