Remove SiteInstanceGroupManager::GetOrCreateGroupForNewSiteInstance

The SiteInstanceGroupManager class currently is mainly used for
handling the default SiteInstance. As we move forward, we don't
necessarily need it and would like to possibly remove it.

Right now, SiteInstanceGroups are constructed through the
GetOrCreateGroupForNewSiteInstance function, but the
SiteInstanceGroup constructor can be called directly instead.

Bug: 1291351
Change-Id: I4a16d6400aedc97cb2291ecc8f3a218ce5949c3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4656805
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1173267}