[MPArch] RFH-FTN separation - simple parent-child fixes

Fix the remaining simple cases where the FrameTreeNode of the
parent RenderFrameHost is accessed when the RenderFrameHost itself
can be used instead.

Bug: 1179502
Change-Id: I4930124e003582ebf25fced2add822be72dc192d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3162439


Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Harkiran Bolaria <hbolaria@google.com>
Cr-Commit-Position: refs/heads/main@{#923329}