Tagged PDFs: Refactor MCID association into helper function

Extract marked content ID (MCID) association logic from
GetStructureSubtree into a private helper method
AssociateMarkedContentWithStructureElement. This is a pure refactoring
with no functional changes.

This prepares for a fix to properly handle structure elements with
multiple MCIDs at different indices, in a follow-up CL.

Bug: 40707542
Change-Id: I517a8d8a8f8aa0882a0a712dd6dd70bccc50128c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7209648
Auto-Submit: Tiago Vignatti <tvignatti@igalia.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1552963}