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This is a refactoring that
1) Avoids using the spare RenderProcessHost for Top Document Isolation
   (TDI) default subframe process.
and
2) Because of (1) simplifies SpareRenderProcessHostManager so that it
   can avoid considering StoragePartition when managing the spare
   process.  Note that after (1) the only caller of
   RenderProcessHostImpl::CreateOrUseSpareRenderProcessHost
   (RenderProcessHostImpl::GetProcessHostForSiteInstance) would always
   pass |nullptr| as the partition argument.

Bug: 808114
Change-Id: Ic87b9142b9e27ca1e95233a4e6e42cc1e8e89f97
Reviewed-on: https://chromium-review.googlesource.com/991058Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549718}