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Just like in HTML, some nodes in a PDF structure tree need not only the
tag name, but also some attributes.

This change adds some of the most generally useful: alt text for images,
language codes for any element, and various table-related attributes.

Bug: 1039816
Change-Id: Ib9c8a768f594bdd61cf5848befa09af290dafdc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090621Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Auto-Submit: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749068}