Made AXNode::GetPosInSet and GetSetSize const

This CL makes the aforementioned functions on AXNode const,
in order to get rid of a `const_cast` on BrowserAccessibility and
on AXPlatformNodeDelegate after CL:4010922 lands.

While working on CL:4010922 I came across the TODO and since it'll
a useful change for that CL decided, I to go ahead and make the
functions const.

Change-Id: I7076cea2a71371cc7e9a13645465b2dcedc45622
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4014814
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Javier Contreras <javiercon@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1076963}