Fix CanAccessDataForOrigin checks in UnassignedSiteInstanceBrowserTest

UnassignedSiteInstanceBrowserTest.BackToNowAssignedSiteInstance is
using CanAccessDataForOrigin() to check whether future navigations to
certain URLs can be committed in a particular process. This is
actually incorrect, as CanAccessDataForOrigin() is now intended only
to check whether a process can check a particular origin's data, and
we have recently added a different API to check whether a navigation
can be committed, namely CanAccessOrigin() with
AccessType::kCanCommitNewOrigin. This CL fixes the test to use that
API instead. This is a prerequisite for upcoming
CanAccessDataForOrigin enforcement improvements that will use a list
of committed origins.

Bug: 40148776
Change-Id: Ibc2da7fee86a4f5b165228cc7576b5ac671def57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798027
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343922}