RFHM::OnDidChangeCollapsedState checks SiteInstanceGroup

This method uses and looks up the parent frame's RenderFrameProxyHost,
which uses SiteInstanceGroup to identify the correct proxy. Clean up to
directly use the parent SiteInstanceGroup rather than going through the
parent SiteInstance.

Test: No behaviour change
Bug: 40269084
Change-Id: Ic86fcdedd7e4ea239ba63d481f74273c25f31867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825812


Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349312}