Support adding attributes to PDF document structure nodes.

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: I64a6b36b0b4ec42fd27ae4ad702afce95c95af5d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268878
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
6 files changed