Simplify interface to StructureElementNode.

Use a std::vector of std::unique_ptr for the children, rather than an
error-prone raw pointer.

Use a string for the element type, rather than an enum.

In both cases, both the old and new format are supported and tested
within Skia until Chromium has migrated.

Also add support for Alt and Lang, which are like attributes
but go right on the structure node. There are only a small
handful of attributes that go on the structure node so this
seems like the simplest solution.

Bug: chromium:607777
Change-Id: I4f315685df35dd9dcd8e1bca6d275cbeb8f2c67a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271816
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
3 files changed