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Refactors the code so that default SiteInstances won't be used anywhere
when the kProcessSharingWithDefaultSiteInstances feature is not enabled.
The old code would still allow default SiteInstances in some scenarios
even if the feature was off. This is surprising and more an artifact
of how the feature was incrementally developed instead of intended
behavior.

Bug: 1015882
Change-Id: I5c46487428d33d77f5706e0f7f3b7795251a1212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877909
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709098}