[B4B-D] Make GetParentForNewNodes return BookmarkParentFolder

Update `BookmarkContextMenuController::GetParentForNewNodes` to return
`BookmarkParentFolder`. This CL also updated the logic of this function
to support a `BookmarkParentFolder` that is represented by one or 2
nodes in selection. This is needed as context menu is used in merged
surfaces.

On follow-up CL, selection will be forced to be not empty.

Bug: 369304373
Change-Id: I7bd362a53a956a3e3a29e5f6efb912cca9a65db0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6054023
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Monica Basta <msalama@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1391691}