Handle initial empty document for SiteInstanceGroup and RenderDocument

Before SiteInstanceGroup with RenderDocument enabled, for same-site
navigations, the initial empty document would always be reused. With
SiteInstanceGroup, that's still true, but the function used to check
that, ShouldCreateRenderFrameHostForSameSiteNavigation, was being
called for same-SiteInstanceGroup but cross-SiteInstance navigations.
The function is only valid for same-SiteInstance navigations. Update
the callers to enforce this and add a comment to clarify this function
only works for same-site navigations.

Test: SitePerProcessNotIsolatedSandboxedIframeTest
Change-Id: I13ac26524a9d36ddf86f405d6f29ecc2817227ea
Bug: 40269084, 40615943
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5906885
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1381056}