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This change converts the site_url param to a SiteInfo in preparation
for making a similar change to DoesSiteURLRequireDedicatedProcess().
This is part of an ongoing process to replace site URL usage in
method signatures with SiteInfo. This change also includes a few minor
cleanups related to default SiteInstances. This change does not
introduce any user visible behavior changes.

- Changed CanBePlacedInDefaultSiteInstance() to take a SiteInfo instead
  of site URL.
- Removed allow_default_site_url from GetSiteForURLInternal() and
  updated callers to handle default SiteInstance cases instead.
- Added SiteInfo::CreateForDefaultSiteInstance() and updated code to use
  that instead of doing a complicated path through ComputeSiteInfo().
- Added SiteInstanceImpl::SetSiteInfoToDefault() to make the conversion
  to a default SiteInstance a little more explicit.

Bug: 1085275
Change-Id: Ib65a41201902c48b13b82172304c8afe15b997fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399016
Commit-Queue: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805466}