Adds an interface for the document creator to pass in a tree
of tags indicating the structure of the document, each with a type
(from a predetermined enum of possible types) and a node ID.
It also adds a setNodeId function to SkCanvas so that page content
can be associated with a particular tag. If both the tag tree and
marked content are present, Skia can now output a properly tagged
PDF.
An example program is included. When used properly, the PDF generated
by this patch is valid and the tags are parsed properly by Adobe
Acrobat. It handles many corner cases like content that spans more
than one page, or tags that don't correspond to any marked content, or
marked content that doesn't correspond to any tags.
However, it doesn't implement all of the features of PDF accessibility
yet, there are some additional attributes that can be associated with
some tags that need to be supported, too, in order to properly tag
things like figures and tables.
Bug: skia:8148
Change-Id: I2e448eca8ded8e1b29ba685663b557ae7ad7e23e
Reviewed-on: https://skia-review.googlesource.com/141138
Reviewed-by: Hal Canary <halcanary@google.com>
