backlight OCR: Finds the node where the tree will be stitched by role

Uses the functionality added in
https://chromium-review.googlesource.com/c/chromium/src/+/5457961
to look up the canvas where the OCRed document will be stitched.
Also takes the opportunity to fix a minor bug with the serializer.

R=​​kyungjunlee@google.com

AX-Relnotes: n/a.
Bug: b/303133098
Change-Id: I1283e6f6f099b9bf7f58f1f9ce3afbda3e728f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5490718
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1293519}