Use SiteInstanceGroup in ExecutePageBroadcastMethod

In RenderFrameHostManager::ExecutePageBroadcastMethod, replace uses of
SiteInstance with SiteInstanceGroup.

Test: No behaviour change
Bug: 1261963
Change-Id: I88215f4ee2f44bafb322b75dfaf3c9271cd6a526
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4553487


Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147617}