[Storage Key] Use IsThirdPartyStoragePartitioningEnabled

This isn't a big deal, but when we can rely on the one function we
should for simplicity and code trace. The only other non test spot is in
components/content_settings/core/common/cookie_settings_base.cc but that
can't change due to a circular dependency on blink if added.

Change-Id: If4e98eb63ce4eccade287b605f8a0210ab5a3615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3973478
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Ari Chivukula <arichiv@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Auto-Submit: Ari Chivukula <arichiv@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1062822}