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Previously we used an enum to select the structure element node type
when building a tagged PDF. Update this to set the node type by name
(as a string), so that Skia doesn't need to maintain the set of all
possible node types.

Also, export the appropriate specific type for headings with heading
levels where possible, just like in HTML (H1 through H6). This
requires plumbing through one additional accessibility attribute
from blink_ax_tree_source.cc.

Bug: 1100698, 1039816
Change-Id: I8189d6dac41c29cbe8662076b752a9b44901e5c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276580
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784456}