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Add new targets to libxml.gn that were added in
 https://chromium-review.googlesource.com/c/chromium/src/+/1894877
Adjust includes to use system libxml headers too

Bug: 1043042
Change-Id: I948c063e212e49b9e7f42fed2b8bf7f4af042ca7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2007110Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735957}