Re-land: Support adding attributes to PDF document structure nodes.

Originally landed: https://skia-review.googlesource.com/c/skia/+/268878
Reverted: https://skia-review.googlesource.com/c/skia/+/271858

The issue was with compilation when PDF support is disabled. See
the diff between patchsets 1 and 2.

This is an important part of writing a tagged PDF. Many of the nodes
in the document structure tree need additional attributes, just like
in HTML.

This change aims to add support for a few useful attributes, not to
be comprehensive.

Bug: chromium:1039816
Change-Id: I15f8b6c41d4fdaa4b6e21775ab6d26ec57eb0f5d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271916
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
7 files changed